Preheat the oven to 325 F° and grease a 9-inch cake pan or springform pan with butter.
Brown ½ cup butter in a small saucepan: Melt the butter over medium-high heat and whisk constantly. After 7-8 minutes, the butter will begin to foam and the solids will begin to turn golden. Take the butter off the heat and continue to whisk for 2-3 minutes, until all the steam escapes, and whisk in the mascarpone cheese and sugar. Transfer the mixture into a large mixing bowl.
Add egg, milk, salt, and vanilla to the bowl. Whisk to combine until the batter appears homogeneous and creamy. Add the baking powder and whisk again, ensuring it has completely dissolved into the batter. Finally, add the flour and mix gently until no streaks of flour remain.
Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until the top and edges of the cake are a deep golden brown. Check your cake 20 minutes into baking—if the top is already golden brown, place a loosely tented sheet of aluminum foil over the top of your cake, protecting the surface from the oven’s heat source. Remove the cake from the oven when the top is stable and almost springy to the touch and the sides of the cake have released from the pan. Let the cake cool completely on a wire rack before slicing or assembling with toppings.
Use an electric mixer on high speed to beat the mascarpone cheese, sugar, salt, and lemon zest together. After about two minutes, the mixture should be creamy and smooth.dd the heavy cream and beat until stiff peaks form.
Mix the fresh berries, lemon juice, sugar, and salt together in a small bowl and let sit for 20 minutes. Serve the cake with dollops of mascarpone whipped cream and spoonfuls of macerated berries.