In a small bowl, add the instant coffee and the milk or cream. Stir together to dissolve the coffee, then set aside.
In the bowl of an electric mixer, fitted with the whisk attachment, beat butter (on medium speed) until completely smooth (30 seconds to a minute).
Add cream cheese, and then continue to beat until smooth.
One cup at time, add 2 cups of powdered sugar, beating after each addition.
Add the vanilla, salt, and the bowl of dissolved coffee.
Add the remaining 2 cups powdered sugar, one at a time.
Continue to blend until all ingredients are fully incorporated and the mixture is smooth and fluffy. Scrape down the sides of the bowl as needed.
Fill a pastry bag with the frosting and pipe onto fully cooled cupcakes.
Top frosted cupcakes with sprinkles or a few chocolate covered espresso beans, if desired.