• Preheat the oven to 350°F. Prepare three 8-inch round baking pans by lining the bottoms with parchment paper and greasing the sides.
• In a large mixing bowl, combine the granulated sugar, vegetable oil, eggs, and lemon extract. Beat until well mixed and creamy.
• Add the lemon zest, lemon juice, and sour cream. Stir until fully combined.
• In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add this to the batter, alternating with the buttermilk, mixing gently until smooth.
• Gently fold in the blueberries.
• Divide the batter evenly among the prepared pans. Bake for 20 to 23 minutes until a toothpick inserted comes out clean.
• Let the cakes cool in the pans for about 10 minutes, then transfer to wire racks to cool completely.
• Chill your mixing bowl. Beat the mascarpone cheese until smooth, then gradually add the heavy whipping cream, powdered sugar, and vanilla extract until stiff peaks form.

Assembling the Cake

• Level the tops of the cakes if necessary. Layer frosting and lemon curd between the cake layers, then frost the top and sides with remaining frosting.
• Decorate with additional frosting, lemon slices, and fresh blueberries.
• Refrigerate the assembled cake until ready to serve.