In a mixing bowl, combine the room temperature butter and vegetable shortening. Beat using an electric mixer until fluffy, about 1 minute.
Add the instant coffee granules dissolved in milk to the butter mixture. Mix until well incorporated.
Gradually add additional instant coffee granules, about 1 tsp at a time, until you achieve your desired coffee flavor. Mix well after each addition.
Begin adding the powdered sugar to the mixture, about 1 cup at a time. Start by mixing the powdered sugar in by hand to avoid a mess, then switch to the electric mixer. Continue adding powdered sugar until the frosting reaches your desired taste, texture, and consistency. The frosting should be smooth, creamy, and easily pipe-able.
Once the frosting is well mixed and has reached your desired consistency, it’s ready to use! Spread or pipe the Coffee Frosting onto your favorite desserts and enjoy the delicious coffee-infused flavor.