Preheat your oven to 350°F (177°C). Grease a 9x13-inch cake pan and line it with parchment paper, leaving an overhang on the sides to lift the cake out easily.

In a large mixing bowl, whisk the cocoa powder with 1 cup of hot brewed coffee until smooth.

To the cocoa mixture, add the oil, granulated sugar, eggs, sour cream, and vanilla extract. Whisk until everything is fully combined.

Gently fold the flour, baking powder, baking soda, and salt into the wet ingredients using a rubber spatula, mixing until the flour is just incorporated.

Transfer the cake to a wire rack to cool completely while you make the frosting.

In a large bowl using a hand or stand mixer, beat the cold mascarpone, vanilla extract, and a pinch of salt on medium-low speed until smooth and creamy.

Slowly add the powdered sugar, mixing on low speed until incorporated.

With the mixer on low, gradually drizzle in the heavy whipping cream and Irish cream. Once combined, increase the speed to high and beat until the frosting becomes fluffy and holds stiff peaks.

Once the cake is completely cool, poke small holes all over the top with a fork. Brush the surface with a few tablespoons of extra coffee, letting it soak in.

Spread the mascarpone frosting evenly over the cake, then dust with cocoa powder. Slice into squares and serve.