For the Cake:

Step 1: Preheat the oven to 350°F (175°C). Prepare three 8-inch round baking pans by lining the bottoms with parchment paper and greasing the sides. If desired, use bake-even strips wrapped around the pans to promote even baking and prevent doming.

Step 2: In a large mixing bowl, combine the sugar, vegetable oil, eggs, and lemon extract. Beat on medium speed until the eggs and oil are well incorporated and the batter is lighter in color, about 2 minutes. Scrape down the bowl as needed to ensure all ingredients are evenly combined.

Step 3: Zest 2 lemons directly into the batter, capturing all those fragrant oils. Then, juice the lemons to obtain ¼ cup of fresh lemon juice. Add the lemon zest, lemon juice, and sour cream to the batter. Beat until well combined, ensuring the lemon flavors are evenly distributed throughout.

Step 4: In a separate bowl, whisk together the dry ingredients: flour, baking powder, and salt. This ensures that the baking powder is evenly distributed, resulting in a cake that rises uniformly.

Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin by adding half of the dry ingredients to the batter, beating on low speed while pouring in half of the buttermilk. Mix just until the flour starts to incorporate. This prevents overmixing, which can lead to a tough cake. Add the remaining dry ingredients and remaining buttermilk, beating until all ingredients are well combined. Be sure to scrape down the sides of the bowl to ensure even mixing.

Step 6: Use a spatula to scrape down the sides of the bowl and stir the batter from the bottom to the top to ensure it's well mixed. This ensures that no pockets of unmixed ingredients remain.

Step 7: In a separate small bowl, toss the blueberries with 2 tablespoons of flour. This helps prevent the blueberries from sinking to the bottom of the cake during baking. Gently fold the blueberries into the batter, saving a small handful to drop on top of the cake batter.

Step 8: Divide the batter evenly between the three prepared pans, about 1 2/3 cups of batter per pan. Drop the remaining blueberries on top of the batter in each pan. This ensures that the blueberries are evenly distributed throughout the cake.

Step 9: Bake at 350°F (175°C) for 20-23 minutes. Rotate your pans in the oven halfway through baking to ensure even browning. Test the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean or with a few moist crumbs, the cake is done.

Step 10: Remove the cakes from the oven and allow them to cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.

For the Frosting:

Step 1: Place the mixing bowl and whisk attachment in the freezer for 5 to 10 minutes to chill. This helps the mascarpone and heavy cream whip up more quickly and easily.

Step 2: Spoon the cold mascarpone cheese into your chilled bowl and beat on a medium-low speed (speed 3 on your stand mixer). Slowly pour in the heavy cream, allowing the mascarpone cheese to turn to a liquid consistency. This slow addition of the heavy cream prevents the mascarpone from clumping.

Step 3: Increase the mixing speed to high (speed 8-10) and beat until soft peaks form. Be careful not to overbeat, as this can cause the mascarpone to curdle.

Step 4: Add the powdered sugar and vanilla extract, beating on low until the sugar is incorporated. Then continue beating at high speed until stiff peaks form. This will give you a stable and beautiful frosting.

Assembly:

Step 1: Use a cake leveler or a serrated knife to cut the domes off the top of the cake layers, creating a flat surface for stacking.

Step 2: Place a dollop of frosting on your cake board or serving plate to secure the bottom layer of cake. Place the bottom layer of cake on top of the frosting.

Step 3: Place half of the mascarpone whipped cream in a large piping bag fitted with a large open round tip, or simply cut off the end of the piping bag. Pipe a layer of frosting onto the cake layer and spread evenly. Then spread half of the lemon curd over the mascarpone filling.

Step 4: Add the second layer of cake and repeat the above steps using all of the remaining lemon curd.

Step 5: Add the final layer of cake on top.

Step 6: Use your piping bag to fill in any gaps between your layers and to create a crumb coat. A crumb coat is a thin layer of frosting that seals in any crumbs and creates a smooth surface for the final frosting layer.

Step 7: Using an offset spatula or icing smoother, wipe away the excess frosting, leaving you just enough to coat the outside of the cake. Do not mix your crumb coating frosting back into your icing if it has cake debris in it.

Step 8: Proceed to frost the top and sides of the cake using your offset spatula. Once the sides of the cake are covered, use an icing smoother to remove any excess frosting and create a smooth finish. You can take the end of your offset spatula and gently place them against the sides of the cake as you rotate your cake to add a little texture.

Step 9: Use the remaining frosting to pipe florets on the top of the cake using an Ateco 809 tip (large open round) or your favorite piping tip. Garnish with lemon slices and leftover blueberries for a beautiful and elegant presentation.

Step 10: Refrigerate the cake for at least 30 minutes to allow the frosting to set before serving. This will make it easier to slice and serve.