For the Cake: 

1. Preheat the oven to 350°F. Prepare three 8-inch round baking pans, line the bottom with parchment paper, grease the sides and dust with flour.
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. After sifting the remaining dry ingredients, gradually add half of the dry mixture to the batter, followed by half of the liquid ingredients (buttermilk and lemon juice). Mix on low speed until all ingredients are incorporated. Then, use a spatula to thoroughly mix the batter, ensuring even distribution of ingredients
4. Toss the blackberries with 2 tablespoons of flour. If the blackberries are large, cut them in half, then gently fold the blackberries into the batter with a spatula.
5. Divide the batter evenly between the three pan, about 1 ¾ cups of batter per pan. 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, the 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 while slowly pouring in about 1/4 cup of the heavy cream, allowing the mascarpone cheese to turn to a liquid consistency. Then add the remaining heavy cream and increase the mixing speed to high, beating it until soft peaks form.
3. Next, add the powdered sugar and lemon extract, beating on low until sugar to incorporate, 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 starting from the outside edge and working towards the middle. Spread evenly with a knife if needed. 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.
4. Use the piping bag to fill in any gaps between the 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.