Preheat your oven to 335°F (170°C). Prepare three 8-inch cake pans by greasing them with butter and lining the bottoms with parchment paper.

In a stand mixer bowl, combine the flour, baking powder, sugar, and salt. Mix on low speed until evenly combined for about 1 minute.

Add the softened butter to the dry ingredients and mix on medium speed until it resembles coarse crumbs, about 2 minutes.

In a separate bowl, whisk together the buttermilk, eggs, and vanilla extract. Gradually add one-third of this mixture to the dry ingredients and mix until combined. Pour in the remaining wet ingredients and mix on medium speed for 2 minutes.

Divide the batter evenly among the prepared cake pans and smooth the tops. Bake for approximately 30 minutes.

Remove the cake layers from the oven and cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

In a large bowl, whip the mascarpone cheese, powdered sugar, and vanilla until smooth. Add the heavy cream and continue to whip until soft peaks form.

In a saucepan, combine granulated sugar, water, and berry jam. Heat on low, stirring until the sugar dissolves and thickens slightly, about 5 minutes.

On your serving plate, place one cake layer and brush it with berry syrup. Spread a layer of mascarpone frosting, then add chopped mixed berries. Repeat with the second layer before topping with the final cake layer.

Frost the top and sides of the assembled cake with the remaining mascarpone frosting and decorate with fresh mixed berries before chilling until ready to serve.