Ina’s Great Chocolate Frosting

Chop chocolate and melt in a heat-proof bowl over pan of simmering water, stirring until just melted. Set aside to cool to room temperature. In bowl of stand mixer fitted with whip attachment, beat butter on medium-high speed until light and fluffy (2-3 minutes). Add egg yolk and vanilla, and continue beating for 3 minutes. Turn mixer off, add powdered sugar in three additions, mixing on low to incorporate, and scraping down sides after each addition. Dissolve coffee powder in 2 teaspoons of hot tap water. With mixer on low, add chocolate and coffee and mix until blended. Spread immediately on cooled cake.