Preheat your oven to 350°F (175°C) and grease a 9x13 baking dish.

Melt ½ cup of unsalted butter and 2 tablespoons of espresso in a saucepan over low to medium heat.

Stir in ¾ cup of cocoa powder and 4 ounces of chopped dark chocolate until melted and smooth.

Combine 1 cup of light brown sugar, ½ teaspoon of kosher salt, and 1 teaspoon of bourbon vanilla extract in a bowl, then pour in the chocolate mixture.

Add 2 large eggs and 2 egg yolks, whisking until glossy.

Sift in 1 cup of all-purpose flour and ½ teaspoon of baking soda, gently folding until just combined.

Pour the batter into the prepared baking dish and bake for 22–25 minutes.

Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Brown ½ cup of unsalted butter in a saucepan until nutty brown, then remove from heat.

Mix the browned butter with 2 cups of powdered sugar, 1 teaspoon of vanilla extract, a pinch of kosher salt, and 2 tablespoons of heavy cream until smooth.

Spread the brown butter frosting over the cooled cake.