Line a standard-sized muffin pan with 9 muffin papers.
Melt the butter, and stir in the sugar and graham cracker crumbs. Divide evenly amongst the 9 muffin papers, pressing gently into the bottoms using your fingers or a spoon.
In a large bowl, use a hand-held mixer to beat the cream cheese until smooth.
Add the Greek yogurt, sugar, egg, key lime zest and juice, and vanilla. Beat for a few minutes until completely incorporated.
*Optional* Add one drop of green food coloring and blend until color is even.
Spoon about 1.5-2 tablespoons of filling over the muffin papers, or until all the batter is used.
Bake in the oven for 25 or so minutes, until centers are set (they may have a very slight jiggle still).
Allow to cool completely, then place in the fridge and chill.
Notes
Store in the fridge in an airtight container for up to one week.