Line 2 cupcake pans with 12 paper muffin cups each. Sift together the flour and baking powder and stir in the polenta and salt.
In a mixer with the paddle attachment, cream together the butter and the sugar until light and fluffy. With the mixer running add the egg yolks, 1 at a time. Beat in the milk, vanilla, and lemon zest. With the mixer on low speed add the dry ingredients to the butter mixture until just incorporated.
In a clean, dry bowl whip the egg whites until soft peaks form. Add a pinch of sugar while the whites are mixing. Add half of the whites to the mixture and beat with the paddle. Remove the bowl from the mixer and gently fold in the remaining egg whites with a spatula to incorporate air and lighten the batter.
Spoon the batter into the lined muffin molds to fill the cups about 2/3 full.
Bake 20 to 25 minutes, or until the tops are golden and a wooden skewer comes out clean when inserted into the center. Cool the entire pan on a wire rack for a few minutes before removing the individual cupcakes. Cool the cupcakes completely to room temperature before frosting.
Combine mascarpone with the confectioners' sugar and beat together with a whisk until combined. Ice the cupcakes and decorate with the berries.