Step 1

Preheat the oven to 375°F (190°C). Grease a 9 x 13-inch baking pan with nonstick spray.

Step 2

Separate the eggs, adding the whites and yolks to two separate large bowls.

Step 3

To the bowl with the yolks, add the granulated sugar and vanilla and beat with an electric hand mixer on medium speed until the mixture is thick and pale, about 3 minutes.

Step 4

Sift in the flour, baking powder, and salt and mix until about halfway incorporated. Add the whole milk and continue to beat until smooth.

Step 5

Clean and dry the hand mixer beaters well, then use the mixer to whip the egg whites on high speed until stiff peaks begin to form, about 3 minutes.

Step 6

Gently fold the egg whites into the batter with a rubber spatula until just incorporated and no white streaks remain.

Step 7

Pour the batter into the prepared baking dish and smooth the top. Bake for 20 minutes, until a toothpick inserted into the center comes out clean. Remove the cake from the oven and let cool for 1 hour.

Step 8

Meanwhile, make the tres leches mixture: In a medium heatproof bowl or liquid measuring cup, whisk together the whole milk, evaporated milk, condensed milk, instant coffee, and rum, if using, until combined. Warm in the microwave for 45–60 seconds, then whisk again until the instant coffee is completely dissolved. Refrigerate until ready to use.

Step 9

Make the mascarpone topping: In a large bowl, whip together the mascarpone and powdered sugar with an electric hand mixer on medium speed until smooth, about 2 minutes. Add the heavy cream, one-third at a time, and whip until light and fluffy, about 3 minutes total. Cover the bowl with plastic wrap and refrigerate until ready to use.

Step 10

Once cooled, pierce the cake all over with a fork. Evenly pour the tres leches mixture over the cake and allow to soak in for 20 minutes.

Step 11

Spread the mascarpone topping on top of the cake, then dust with the cocoa powder. Cover the baking dish with plastic wrap and refrigerate for 6–8 hours.

Step 12

Slice the cake and serve with berries, if desired.

Step 13

Enjoy!