Prepare the Cakes: Preheat the oven to 375°F (190°C) with a rack in the center position. Line a standard 12-cup muffin pan with 11 paper liners. Set aside. In a medium bowl, whisk together the all-purpose flour, cornmeal, granulated sugar, baking powder, and salt. Set aside.
In a stand mixer, fitted with a paddle attachment, combine the whole eggs and softened butter (yes, I know it sounds a bit weird, trust me on this!). Add all of the dry ingredients to the bowl. Turn on the mixer to medium speed (4) and beat the batter for 2 minutes, or until all of the ingredients are evenly incorporated and the batter is smooth.
Divide the batter evenly among the lined muffin cups. The batter should reach about half-way up each cup. Bake for 14 to 16 minutes, or until the cornmeal cakes are golden brown and springy to touch. Remove from the oven and allow to cool in the baking tin, on a rack, for 3 to 5 minutes. Remove the cornmeal cakes from the tin, set on a rack, and allow to cool to room temperature before decorating.
Prepare the Whipped Mascarpone: In a clean stand mixer bowl, fitted with a paddle attachment, combine the mascarpone cheese, heavy cream, powdered sugar, and vanilla extract. Beat at low speed, slowly increasing to medium speed, until the mixture thickens and forms soft peaks. Note: Be careful to avoid over-beating the mixture, it comes together very quickly.
Assembly: Using a spoon, add a dollop of whipped mascarpone onto the center of each cooled cornmeal cake. Using a fine-meshed sieve, dust the cakes lightly with powdered sugar. Top each cake with a mixture of fresh blackberries and fresh raspberries, and freshly grated lemon zest. Serve immediately.
Frosted cakes can be stored in an airtight container in the fridge for 1 to 2 days, but are best served at room temperature and the day they are baked. If you would prefer to skip the mascarpone cheese, increase the heavy cream quantity to 1 cup.