In a large bowl, combine the egg whites and cream of tartar. Set aside.
In a saucepan, bring the sugar and coffee to a boil. Simmer until a candy thermometer reads 240°F (116°C). Watch out for overflowing. Remove from the heat.
Beat the egg whites and cream of tartar with an electric mixer on medium speed until frothy. Drizzle the hot syrup onto the egg whites, avoiding the mixer’s beaters. Beat continuously at high speed until completely cooled and stiff peaks form.
Add the butter, 1 or 2 cubes at a time, beating until smooth and creamy. With a spatula, scrape the bottom and sides of the bowl several times during this step.