Preheat the oven to 350°F (180°C). Lightly grease a 9-inch springform pan and line the bottom with parchment paper.

In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, cornstarch, sugar, baking powder, and salt. Mix on low speed until everything is evenly combined.

Add the softened butter and beat on medium-low for 3-5 minutes. The mixture will look like wet sand.

In a separate bowl, whisk together the oil, eggs, vanilla extract, buttermilk, and whole milk until smooth.

With the mixer on low speed, slowly pour the wet ingredients into the dry mixture. Scrape down the sides of the bowl as needed and mix until just combined. The batter will be thin, which is normal.

Pour the batter into the prepared springform pan and smooth the top with a spatula. Bake for 25-35 minutes, or until the top is lightly golden and a toothpick inserted in the center comes out with a few moist crumbs.

Let the cake cool in the pan for 15 minutes, then run a knife around the edges and release the springform sides. Cool completely on a wire rack.

Add the cold mascarpone, vanilla, and a pinch of salt to the bowl of a hand mixer or stand mixer fitted with the whisk attachment. Mix on medium-low speed until the mixture is smooth and creamy.

Slowly add the powdered sugar, mixing on low to start. Once it's mostly incorporated, increase the speed to medium-high and beat for 1-2 minutes. Pause to scrape down the sides of the bowl as needed.

With the mixer on low, gradually pour in the cold heavy cream. The frosting will look soft and slightly runny at first.

Turn the speed up to high and whip for another 1-2 minutes until the frosting thickens and holds stiff peaks, just like whipped cream.

Spread the frosting on top of the cake and add the mixed berries. Slice and enjoy!