Place the butter in the bowl of a mixer and beat on medium speed for 5 minutes, scraping down sides and bottom. Don't stop at a couple of minutes and think that's enough. By 5 minutes, the frosting will lighten up in color and texture and look fluffy.
In a microwavable measuring cup, heat water and oil until warm (about 20 seconds)
Add in the coffee granules.
Whisk until smooth and the coffee is rehydrated.
Next, add in a cup of powdered sugar and whisk until smooth.
Add in the remaining powdered sugar and add a tablespoon of water at a time as needed, until you whisk it and it looks like a pourable sauce.
Then, drizzle in the coffee mixture slowly as the mixer is on low speed.
Add up to an additional cup of powdered sugar if you want it sweeter.
Mix on medium speed for another minute so it's all fluffy, blended, and smooth, and the air bubbles have been pressed out by the paddle blade against the sides of the bowl.
Slather the frosting on the cake and swirl it with the back of a spoon to create swoops and valleys.
Refrigerate the cake with frosting until fully set and firm. Remove cake an hour or two before serving.