Preheat an oven to 425 degrees Fahrenheit and line a muffin tin with paper liners.
In a large bowl, use a hand mixer to beat together melted butter with sugar, about 1 minute. Add in eggs and vanilla extract and beat until just combined.
Add in pistachio pudding mix, baking powder, salt, and milk. Mix to combine. Add in all-purpose flour and mix until flour has been incorporated.
Divide batter amongst each muffin cup, filling 3/4 full. Sprinkle tops with sugar crystals (optional).
Bake in the preheated 425 degree oven for 7 minutes. After 7 minutes, reduce the heat in the oven to 350 degrees F and continue baking 10 minutes, until muffin is cooked all the way through. Remove from muffin tin and transfer to a wire rack to cool completely.