Preheat the oven to 350ºF. Line a cupcake pan with cupcake liners.

In a small saucepan over low-medium heat, melt unsalted butter and espresso together. Once the butter is completely melted, remove from the heat.

Immediately add the cocoa powder and chopped chocolate to the saucepan. The residual heat will melt the chopped chocolate. Stir together until the chocolate is completely melted. There may be small lumps of cocoa powder still, but they will mix into the batter at the next stage.

In a large bowl, add the light brown sugar, salt, and vanilla.

Add the chocolate mixture to the large mixing bowl and whisk everything together until a smooth batter forms.

Add the eggs and egg yolks and whisk until the batter is glossy and smooth.

Add the flour and baking soda. Whisk until smooth.

Pour the batter into the prepared cupcake pan. Bake for 20-22 minutes, or until a toothpick inserted comes out clean.

Allow the cupcakes to cool completely before frosting.

In a stand mixer, using the paddle attachment, beat the softened butter for 1-3 minutes on high speed, or until the butter is creamy and slightly pale.

While the espresso shot is still hot, add the finely ground espresso beans and mix together.

In a small bowl, add the espresso mix, vanilla, and 1 1/2 cups (180g) of powdered sugar. Whisk everything together until a thick mixture forms.

Add the espresso mixture to the butter and mix together.

Add the kosher salt and the remaining 4 1/2 cups of powdered sugar, one cup at a time, until fully combined.

Frost the cooled cupcakes and enjoy!

Adjustments: If the buttercream is too sweet you can add a pinch of kosher salt to reduce the sweetness. If the buttercream is too thin, you can add more powdered sugar, 1/4 cup at a time. If the buttercream is too thick, you can add heavy cream, one tablespoon at a time.