How to Make It
Preheat the oven to 375°. Butter an 8-inch round cake pan and line with parchment paper. Butter the paper and dust the pan with cocoa powder, tapping out the excess. In a fine-mesh sieve set over a medium bowl, sift the 1/2 cup plus 2 tablespoons of cocoa powder with the flour and baking powder.
In a stand mixer fitted with the whisk, beat the egg yolks at medium-high speed until thickened slightly, about 2 minutes. Beat in 1 cup of the granulated sugar, 1/4 cup at a time, beating well after each addition, then beat until the mixture is very thick and pale yellow, about 3 minutes longer.
Beat in the cubed butter at medium speed, a few pieces at a time, beating well after each addition, then beat until the mixture is fluffy, about 2 minutes longer. Reduce the speed to low and mix in the dry ingredients in 2 additions, alternating with the milk, until smooth. Scrape the batter into a large bowl. Clean the mixing bowl and whisk.
In the stand mixer fitted with the whisk, beat the egg whites at medium speed until thick and foamy, about 2 minutes. Gradually beat in 3 tablespoons of the granulated sugar, beating well after each addition. Beat until the whites are thick and glossy, about 1 minute longer. Using a rubber spatula, mix one-third of the egg whites into the batter until smooth, then gently fold in the remaining egg whites until no streaks remain. Clean the mixing bowl and whisk.
Scrape the batter into the prepared cake pan. Bake for about 45 minutes, until the edge of the torta is firm and a toothpick inserted in the center of the cake comes out slightly wet. The torta will continue to set as it cools. Transfer the pan to a rack to cool for 30 minutes, then turn out the torta onto the rack, peel off the parchment and turn the cake so it’s flat side down. Let cool completely, about 1 hour.
Just before serving, in the stand mixer fitted with the whisk, beat the mascarpone with the cream and the remaining 1 tablespoon of granulated sugar at medium-high speed until soft peaks form. Dust the torta with confectioners’ sugar and serve with the whipped mascarpone and pistachios.