Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.In a medium-sized bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside. In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.Add the eggs one at a time, beating well after each addition. Mix in the lemon zest and lemon juice until well combined. Gradually add the flour mixture to the wet ingredients, alternating with the milk, beginning and ending with the flour mixture. Mix until just combined. Stir in the vanilla extract.
Divide the batter evenly between the prepared cake pans. Smooth the tops with a spatula.
Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean. Remove from the oven and let the cakes cool in the pans for 10 minutes. Then transfer them to a wire rack to cool completely.
While the cakes are cooling, prepare the mascarpone cream frosting. In a mixing bowl, combine the mascarpone cheese, heavy cream, powdered sugar, and vanilla extract. Beat on medium speed until smooth and creamy. Be careful not to overbeat.
In a small bowl, gently toss the mixed berries with granulated sugar (if using) and lemon juice. Set aside.
Once the cakes are completely cooled, place one cake layer on a serving plate. Spread a generous amount of mascarpone cream frosting over the top. Layer the lemon berry filling over the frosting.
Place the second cake layer on top and frost the entire cake with the remaining mascarpone cream frosting. Smooth the frosting with an offset spatula.
Decorate the top of the cake with fresh berries and lemon zest for an elegant touch.Allow the cake to chill in the refrigerator for at least 2 hours to set before serving. Slice and serve this heavenly Lemon Berry Mascarpone Cake, and savor the burst of flavors with every bite!