Set your mixing bowl in a pan of simmering water (for ease of use, use your stand mixer's bowl) and whisk egg whites and sugar for 3 to 5 minutes, until fluffy, marshmallow-like and hot. You may need a separate hand-held electric hand mixer for this.
Remove bowl from heat, scrape the handheld's whisks free of marshmallow meringue, and then using your stand mixer, beat the meringue for another 5 minutes with the whisk attachment.
Add half the butter and beat until smooth. (You may opt to use the paddle attachment for this, or stay with the whisk attachment) Add the rest of the butter, beat until smooth again. Scrape sides, stir buttercream mixture quickly, then go back to beating for 6 to 10 minutes.
Dissolve cocoa, coffee and vanilla extract in a very small amount of hot water, stirring vigorously.
Add to buttercream and beat on high for a further 2-3 minutes, scraping as needed.
Use to put a layer of buttercream between the cakes, and to frost and decorate the cake.