Instructions

For the Cake: 

1. Preheat the oven to 350°F. Prepare three 8-inch round baking pans, line the bottom with parchment paper and grease the sides. Prepare the bake even stripes if desired.
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, scrape down the bowl as needed. Next, add the lemon zest and the sour cream and beat until well combined.
3. Next, zest 2 lemons into the batter and then juice the lemons to get 1/4 cup of lemon juice. Set the lemon juice aside. Add the lemon zest, and sour cream to the wet ingredients and beat until well combined.
4. Sift together the remaining dry ingredients. Alternate adding half the dry ingredients followed by half of the liquid ingredients (buttermilk and lemon juice) and mix on a lemon speed just until the flour starts to incorporate. 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.
5. Toss the blackberries with 2 tablespoons of flour, and add additional if needed. If the blackberries are large, cut them in half, then gently fold the blackberries into the batter.
6. Divide the batter evenly between the three pan, about 1 ¾ cups of batter per pan. Drop the remaining blackberries on top. Bake at 350°F for 28-32 minutes.  Test the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, you cake is done. Remove from the oven and allow to cool completely.

For the Frosting: 

1. Place the mixing bowl and whisk attachment in the freezer for 5 to 10 minutes to chill.
2. Spoon the cold mascarpone whipped cream into your 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.
3. Then increase the mixing speed to high (speed 8-10) and beat it until soft peaks form.
4. Next, add the powdered sugar and lemon extract, beating on low until sugar is incorporated. Then continue beating at high speed until stiff peaks form.

To Assemble 

1. Use a cake leveler to cut the domes off the top of the cake if needed. Place a dollop of frosting on the cake board and place the bottom layer of cake on top of that.
2. Place half of the 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 and spread it evenly. Then spread half of the lemon curd over the mascarpone frosting.
3. Add the second layer of cake and repeat the above steps using all of the remaining lemon curd. Next, add the final layer of cake on top inverted. 
4. Use the piping bag to fill in any gaps between your layers and to create a crumb coat. 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.
5. Proceed to frost the top and sides of the cake using the offset spatula with the remaining frosting. Garnish with leftover blackberries.