1/2 cup soft unsalted butter
6 ounces semisweet chocolate, melted and cooled
1 egg yolk
1/2 teaspoon vanilla extract
1 teaspoon instant coffee powder

Cream butter briefly until light and fluffy. Beat in cooled chocolate, egg yolk, vanilla, and coffee powder. This can be used immediately or refrigerated until needed. It will harden in the refrigerator and should be brought to room tempurature before being used.