Measure egg whites and powdered sugar into a bowl. Using the paddle attachment, beat on medium/high for 5 minutes until it is white and glossy.
Set mixer to medium, gradually add butter making sure all combined before adding next piece.
Once all the butter is incorporated then beat for 5-10 minutes on medium/high until light and fluffy.
Then add the coffee and fold in until fully incorporated.
Once the buttercream is ready, fill a piping bag with a star nozzle on the end and pipe on to the cooled cupcakes.