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

In a small saucepan over low to medium heat, combine unsalted butter and espresso, stirring until melted completely.

Once combined and cooled slightly, stir in the cocoa powder and chopped dark chocolate until smooth.

In a large mixing bowl, combine light brown sugar, kosher salt, and bourbon vanilla extract; add the chocolate mixture and stir until combined.

Add the large eggs and egg yolks, whisking until the mixture is glossy and smooth.

Sift in the all-purpose flour and baking soda, gently folding into the wet batter without overmixing.

Pour the batter into the prepared baking dish and bake for 22 to 25 minutes; check for doneness with a toothpick.

Remove the cake from the oven and cool completely in the pan for about 30 to 40 minutes.

To make the frosting, brown the unsalted butter in a saucepan until bubbling and nutty, then cool slightly.

In a stand mixer, combine the browned butter with powdered sugar, then add vanilla extract, kosher salt, and cold heavy cream, mixing until smooth.

Once the cake is cooled, frost the top generously with the brown butter frosting and add any desired toppings.

Slice and serve your Dark Chocolate Espresso Cake, optionally pairing it with ice cream or espresso whipped cream.