Combine all three ingredients for the Ganache Filling in a bowl. Double boil the chocolate mixture. Whisk the mixture until smooth. Remove from the double boiler and chill in the refrigerator for not more than 25 minutes.
Chocolate Cupcakes
Mix the bittersweet chocolate with the cocoa powder in a bowl. In a cup, pour 3/4 cup of hot water into the instant coffee powder. Slowly add the coffee into the chocolate mixture and whisk until smooth. Chill in the refrigerator for 20 minutes.
Pre-heat the oven to 350°F (176°C). Sift the bread flour, sugar, salt and baking soda in a bowl. Set aside.
Use a new bowl and beat the oil, eggs, white vinegar and vanilla extract together. Slowly add in the chocolate-coffee mixture. Whisk well.
Next, add in the sifted ingredients 2-3 tablespoons each time and whisk until smooth. Divide the batter evenly among the cupcake liners. Drop in a rounded teaspoon of the Ganache Filling into the center of each cupcake batter. Bake the cupcakes for 15-18 minutes until firm to touch.
Vanilla Buttercream Frosting
Use a mixer with a paddle attachment and beat the butter until soft and fluffy at medium speed. Slowly pour in 3 cups of confectioners' sugar, 1 cup at a time, and mix well. Then add the vanilla extract and 1 tablespoon heavy cream while mixing for another 2-3 minutes.
Adjust the icing stiffness accordingly. Add an additional 1 tablespoon of heavy cream if the icing consistency is too stiff, or 1/2 cup of confectioners' sugar if the icing is too thin.
Notes
Alternate method to melt the Ganache Filling is to use the microwave. Combine all ingredients in a microwave-safe bowl and heat on high for 20-30 seconds. Whisk until smooth. Chill in the refrigerator for no longer than 25 minutes.