Preheat oven to 350°F (175°C). Grease and flour cake pans.
Whisk together flour, baking powder, and salt.
Cream butter and sugar until light and fluffy.
Add eggs one at a time, beating well.
Mix in vanilla extract and lemon zest.
Alternate adding flour mixture and milk.
Divide batter evenly into pans and bake 25-30 minutes.
Cool cakes for 10 minutes, then transfer to racks.
Cook berries, sugar, and lemon juice until juices release.
Add cornstarch slurry, simmer until thickened.
Cool berry filling completely.
Beat mascarpone until smooth.
Whip heavy cream with sugar and vanilla until stiff peaks form.
Fold whipped cream into mascarpone.
Level cakes if needed.
Pipe a frosting border on the first layer.
Fill with berry filling and top with second cake.
Frost entire cake with mascarpone frosting.
Decorate with fresh berries and mint.