In a standing mixer fitted with the whisk attachment, beat the butter at medium-high speed until light and fluffy, about 1 minute. Add 2 cups of the powdered sugar and beat for 2 to 3 minutes until fluffy. Add lime zest, lime juice and remaining powdered sugar. Beat for another 1-2 minutes, scraping down the bowl as needed. If desired, add a little green food coloring.

Transfer the key lime pie creme to a piping bag fitted with a large round tip (or a freezer zip-lock bag with the corner snipped off.) Pipe the filling onto half of the graham crackers, and top with the remaining graham crackers, pressing down until the filling reaches the edges.

Enjoy or chill in the fridge for a firmer creme filling.