Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with butter or cooking spray.

In a small saucepan over low to medium heat, melt 1 ½ sticks of unsalted butter with 6 oz of espresso.

Stir in ½ cup of cocoa powder and a ⅓.5 oz bar of chopped dark chocolate into the butter and espresso mixture.

In a large bowl, combine 1 ¼ cups of light brown sugar, ½ teaspoon of kosher salt, and ½ tablespoon of bourbon vanilla extract.

Slowly pour the warm chocolate mixture into the bowl with your dry ingredients and mix everything together until the batter is silky and smooth.

Crack in 3 large eggs and 2 egg yolks into the bowl and whisk until the mixture is glossy and well-combined.

Fold in 1 cup of all-purpose flour and ½ tablespoon of baking soda until just combined.

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

Cool the cake completely in the baking dish on a wire rack.

Melt 2 sticks of unsalted butter in a saucepan over medium heat until it turns golden brown.

In a stand mixer, combine the browned butter with 4 cups of powdered sugar and mix on low speed until blended.

Add in ½ tablespoon of vanilla extract, ½ teaspoon of kosher salt, and 3 tablespoons of heavy cream, and whip until fluffy.

Once the cake is completely cooled, frost the top evenly with the brown butter frosting and serve.