If you would like to brown the butter (which makes the frosting so much more flavorful), melt the butter in a small saucepan over medium-low heat. Let it bubble and boil while mixing frequently and watching carefully, until it begins to brown (it burns very easily). Remove from heat, pour into a heat-proof mixing bowl, add the salt and coffee/espresso powder, and let it cool at room temperature until the bowl feels cool to the touch (it should still be liquid).
If you did not brown the butter, in a very small bowl or cup, dissolve the espresso/coffee powder in about 2 tsps of hot water.
If you did not brown the butter, in a medium-large mixing bowl beat the butter, dissolved espresso/coffee, and salt together until very light and fluffy. Whether you are using browned butter or not, add the cream cheese, vanilla, and maple extract to the coffee butter and beat until well combined. Add ½ cup of powdered sugar and mix on low until combined. Add another ½ cup of powdered sugar and mix to incorporate. Add the final ½ cup of powdered sugar and mix until combined. Scrape the sides and bottom of the bowl with a spatula. Beat the frosting very well until fluffy.