Mocha Buttercream by Marika Kolinsky-Philips
1 c milk
1/2 c powdered sugar
3 T cornstarch
2 T flour
2 egg yolks
3 T sugar
Bring 2/3 c milk to a low boil.
To the rest of the milk, add the rest of the ingredients and stir smooth. Stir this mixture slowly into the boiling milk and bring again to a boil while stirring vigorously and continuously. It will be thick. Remove from heat and cover against the custard surface with plastic wrap. Cool to barely lukewarm.
1/2 # sweet butter, soft
1 scant C. powdered sugar
3 oz melted chocolate
2 tsp instant coffee dissolved in 1 T boiling water and cooled
Beat butter till creamy. Add powdered sugar and continue to beat until fluffy. Now slowly, by spoonful, add the cooled custard while continuing to beat.
Note: Make sure neither the butter nor the custard are too cold. This will separate the mixture. Should this happen, adding more soft butter, a tsp. at a time, will tighten the buttercream.
Beat in the chosen flavors substance(s). Use as directed in your recipe.
I have a great chocolate buttercream recipe that you could alter to make more mocha-y. This is the traditional buttery buttercream, vs. a buttercream with conf. sugar. The recipe is from the Barefoot Contessa, her first book. It already includes some coffee or espresso to increase the chocolate flavor, but you could boost that amount (while decreasing another liquid) to increase the coffee side.
This is the cheater's version I was taught in cooking school where you were too pressed for time to make a real, french buttercream. I no longer have original class notes, but this is the recipe from Professional Baking (Gisslen) that is very similar (I think):
Use a good, strong electric mixer.
Cream together 1/2 # of unsalted butter, 1/4# of shortening, and 1# of confectioners sugar with the paddle leaf until well blended.
Add 1 1/2 egg whites, 1/2 t lemon juice, 1/2 t vanilla. Mix at medium until blended. Switch to high speed until light and fluffy. Use immediately.
For mocha, mix 1 oz of semisweet or bittersweet chocolate, 2 t of instant coffee, and 2 t water. Microwave a few seconds and stir. Repeat until blended and chocolate is melted. Cool slightly. Scoop out 1/4 of finished buttercream into chocolate, mix, then mix back into the rest of the buttercream.
Recipe amounts and flavoring intensity can be altered to taste.