Instructions

for the cake

Adjust oven rack to middle position and heat oven to 325 degrees. Lightly grease bottoms and sides of two 9-inch round cake pans with butter and line with parchment; grease parchment.
Whisk 2 cups (284 grams) all-purpose flour, 2¼ teaspoons baking powder, and ½ teaspoon table salt together in large bowl. Whisk 6 large egg yolks, ¾ cup water, ½ cup vegetable oil, ½ cup (99 grams) granulated sugar, and 2 teaspoons vanilla extract in second large bowl until combined. Whisk dry ingredients into wet ingredients until fully combined.
Using stand mixer fitted with whisk attachment, whip 6 large egg whites on medium speed until very foamy, about 2 minutes. Increase speed to high and gradually add remaining 1 cup plus 1 tablespoon (211 grams) sugar in steady stream. Whip until stiff peaks form, 3 to 5 minutes longer.
Stir one-quarter of egg white mixture into batter to lighten. Using silicone spatula, gently fold in remaining egg white mixture until no white streaks remain.
Divide batter evenly between prepared pans and smooth tops. Bake until golden brown and toothpick inserted in center comes out clean, 40 to 45 minutes. Let cakes cool in pans on wire rack for 15 minutes. Run paring knife around edges to loosen, then invert onto greased wire rack, discard parchment, and invert again. Let cool completely, about 1 hour.

for the coffee syrup

Stir ½ cup (99 grams) granulated sugar into ⅓ cup strong hot coffee until dissolved. Stir in ½ cup Kahlúa; set aside.

for the mascarpone whipped cream

Using stand mixer fitted with clean whisk attachment, beat 2 cups chilled mascarpone and 2 cups chilled cream on medium speed until smooth, about 30 seconds. Add 1 cup (113 grams) confectioners’ sugar, 4 teaspoons vanilla, and ⅛ teaspoon salt, increase speed to medium-high, and whip until medium peaks just form, 1 to 2 minutes longer.

to assemble

Using skewer or toothpick, poke holes all over both cooled cakes. Brush coffee syrup evenly over tops of both layers (use all of syrup, allowing it to soak in as needed).
Place 1 cake layer on on cake turntable or cake stand. Spread 1½ cups mascarpone whipped cream evenly over top of layer. Dust layer lightly with 1 tablespoon cocoa powder. Place second cake layer on top, pressing lightly to adhere. Spread 1 cup mascarpone whipped cream around sides of cake in thin, sheer layer, allowing cake to show through. Transfer remaining mascarpone whipped cream to pastry bag fitted with tip of your choice and pipe decoratively over top of cake.
Refrigerate at least 1 hour. Dust with extra cocoa powder before serving.