Preheat and prep: Heat your oven to 350°F (175°C). Line a 9x9-inch pan with parchment, or use a rimmed baking sheet if you prefer to cut the cake into strips.

Make the sponge batter: In a bowl, whisk 1 1/2 cups almond flour, 2 tablespoons coconut flour, 1 teaspoon baking powder, and 1/4 teaspoon salt. In another bowl, beat 6 tablespoons melted butter with 1/2 cup keto sweetener until combined. Whisk in 4 large eggs and 1 teaspoon vanilla.

Combine and bake: Stir dry ingredients into wet until smooth. Spread the batter evenly in the pan. Bake 15–18 minutes, until set and lightly golden. Cool completely.

Brew the coffee: Prepare 3/4 cup strong espresso or coffee. Stir in 1–2 tablespoons sweetener and 1–2 tablespoons rum or coffee liqueur if using. Let it cool to room temperature.

Make the mascarpone cream: In a large bowl, beat 1 1/2 cups cold heavy cream with 1/3 cup sweetener and 1 teaspoon vanilla to soft peaks. In another bowl, gently loosen 16 ounces mascarpone with a spatula (don’t overbeat). Fold the whipped cream into the mascarpone in two additions until smooth and thick.

Assemble the first layer: Slice the cooled sponge in half horizontally, or cut it into “ladyfinger” strips. Place a layer in an 8x8 or 9x9 dish. Brush or spoon the coffee mixture over the cake until moist but not soggy.

Add cream: Spread half the mascarpone cream evenly over the soaked layer.

Second layer: Add the remaining sponge on top. Soak again with the coffee mixture. Spread the rest of the mascarpone cream over the top, smoothing the surface.

Dust and chill: Sift unsweetened cocoa powder generously over the top. Chill for at least 4 hours, preferably overnight, to set the layers and develop flavor.

Serve: Slice with a clean, sharp knife. Wipe the blade between cuts for neat squares. Add a light final dusting of cocoa if needed.