I am off to the grocery store, here is the birthday cake recipe I ended up with:

Espresso brownies:
Nonstick vegetable oil cooking spray
1/3 cup plus 2 tablespoons water
1/3 cup vegetable oil
2 large eggs
2 tablespoons plus 2 teaspoons espresso powder
1 (19.8-ounce) box brownie mix (recommended: Duncan Hines)
3/4 cup semisweet chocolate chips
1 teaspoon vanilla extract
1 1/2 cups powdered sugar
1 tablespoon unsalted butter, room temperature
Preheat oven to 350 degrees F.

Spray a 9 by 13-inch baking pan with nonstick spray. Whisk 1/3 cup of water, oil, eggs, and 2 tablespoons espresso powder in a large bowl to blend. Add the brownie mix. Stir until well blended. Stir in the chocolate chips. Transfer the batter to the prepared baking pan. Bake until a toothpick inserted into the center of the brownies comes out with a few moist crumbs attached, about 35 minutes. Cool completely.

Meanwhile, dissolve the remaining 2 teaspoons of espresso powder in the remaining 2 tablespoons of water in a medium bowl. Whisk in the vanilla. Add the powdered sugar and butter and whisk until smooth. Pour the glaze over the brownies. Refrigerate until the glaze is set. Cut into bite-size pieces. Arrange the brownies on a platter and serve.

Mascarpone Filling:
1 1/2 cups heavy whipping cream
1/3 cup sugar
2 teaspoons vanilla extract
1 pound mascarpone cheese, softened to room temperature
For the filling, whip cream with sugar and vanilla until soft peaks form. Fold cream into softened mascarpone.

For Icing:
1 stick butter, softened
4 cups powdered sugar, divided
1 1/2 teaspoons instant espresso powder
1/2 cup chocolate syrup

I feel my ass growing as I type this. Must go to gym!