Preheat the oven to 350°F. Prepare a muffin tin with cupcake liners (I like to use parchment liners).
In a large bowl, sift together the cocoa powder, flour and baking powder. Add sugar and salt into the bowl as well.
Whisk until combined.
In a separate medium bowl, whisk together the butter, eggs, beer and vanilla extract.
Add the wet ingredients into the dry ingredients in two batches. Mix until the dry ingredients are fully moistened after each batch, ensuring to scrape down the sides and bottom of the bowl.
Using a large spoon or cookie scoop, scoop the batter into the cupcake liners filling them about ¾ of the way.
Bake for about 20-25 minutes until a toothpick inserted into the center comes out clean. Allow to fully cool on a wire rack.
While the cupcakes are baking, make the ‘coffee butter’. Melt the 2 tablespoons of butter in a small bowl. Then, add the espresso powder. Stir for 2 minutes until dissolved. If there are still some undissolved granules, you can strain the mixture in a sieve into a new bowl.
Place the ‘coffee butter’ in the fridge to chill.
Place the egg whites and sugar into the bowl of a stand mixer.
Set the bowl over a pot of simmering water (do not let the water touch the bowl). Whisk the sugar and eggs together and continue to whisk until the sugar dissolves.
Check that the sugar/egg mixture reaches 165° or the sugar has fully dissolved by rubbing the mixture between your fingers. If there’s no grit, it’s ready!
Set the bowl in the stand mixer with a whisk attachment.
Beat on the highest setting for about 10-15 minutes or until the bowl is fully cool to the touch.
With the mixer still on high, add the butter, one tablespoon at a time. Add the ‘coffee butter’ as well.
Continue to beat the frosting until all the butter is worked in and it is smooth and creamy. Add the vanilla and mix.
Pipe or spread the frosting onto the cupcakes. I used a piping bag fitted with a Wilton 2D tip.