Preheat oven to 350°F (180°C). Grease two 8-inch round cake pans and line bottoms with parchment paper.
In a large bowl, combine granulated sugar and lemon zest. Rub together with fingertips for 1 minute until fragrant and moist.
Add cake flour, baking powder, and salt to sugar mixture. Whisk until evenly combined.
In separate bowl, mix whole milk, sour cream, and oil until smooth. Add egg and vanilla extract, whisking until fully incorporated.
Pour wet mixture into dry ingredients. Stir with spatula until just combined. Divide batter evenly between prepared pans.
Bake for 22-25 minutes until toothpick inserted in center comes out clean. Cool in pans 10 minutes, then turn out onto wire rack to cool completely.
For frosting, beat mascarpone cheese in large bowl until smooth and creamy, about 2 minutes.
In chilled bowl, whip heavy whipping cream, powdered sugar, and lemon juice until stiff peaks form.
Gently fold whipped cream into mascarpone in three additions, being careful not to deflate mixture.
Place one cake layer on serving plate. Spread one-third of frosting over top. Add second layer and frost top and sides with remaining frosting.
Top generously with mixed fresh berries and fresh mint leaves if desired. Chill 30 minutes before slicing.