Before blending the cake, place the blackberries, about 40 of them, half to 2/3rds of the bag, on a plate or tray lined with 2 layers of paper towels. Allow them to thaw for 30-45 minutes to release some of their excess liquid.
Preheat oven to 375 degrees.
Line the bottom of an 8-inch cake pan with parchment paper and cooking spray.
In a food processor, finely grind the hazelnuts with the sugar and salt. Blend until a fine “hazelnut sugar” has been created, about 60-90 seconds.
Add butter, vanilla, and eggs to the food processor and blend until smooth.
Pulse in the flour into the mixture just until moistened about 10-15 seconds, being careful not to over-mix.
Place the batter in the prepared pans filling them about three-quarters full.
Drop Marion blackberries into the batter, 1 berry every 1-2 inches.
Sprinkle the chopped hazelnuts across the top of each cake.
Bake, rotating halfway through, until golden brown, about 40-45 minutes.
Transfer to a wire rack and allow to cool for 10 minutes. Turn the cake out of the pan and allow to cool completely on the wire rack.
Fold together the mascarpone, sugar, and vanilla bean paste until well combined.
Serve the cake with a generous scoop of the vanilla mascarpone.