In a large mixing bowl, combine all-purpose flour, granulated sugar, baking powder, and salt. Whisk until blended. Add softened unsalted butter and mix until coarse crumbs form.

In another bowl, whisk together eggs, buttermilk, and vanilla extract until creamy. Ensure they are at room temperature.

Gradually pour wet ingredients into the dry mixture, stirring gently until just combined.

Preheat oven to 335°F (170°C). Divide batter among three greased 8-inch cake pans and smooth tops. Bake for 30 minutes or until a toothpick comes out clean. Cool in pans for 10 minutes then transfer to wire racks.

For frosting, beat mascarpone cheese with powdered sugar and vanilla until smooth. Whip heavy cream to firm peaks and fold into the mascarpone mixture.

For simple syrup, combine water, granulated sugar, and berry jam in a saucepan. Simmer for 3 minutes, then cool.

To assemble, layer cooled cakes on a platter, brushing each with simple syrup, adding frosting, and topping with berries. Repeat for layers and finish with frosting and extra berries on top.