Preheat oven to 375˚F. Spray a 10 x 15 jelly roll pan or a sheet pan generously with baking spray and rub with a paper towel to coat all surfaces. Set aside.
In a medium-large microwave-safe bowl, combine the cubed butter and water. Microwave on high for 2 minutes. Most of the butter should be melted. Remove from the microwave and stir until the butter is completely melted.
Add the buttermilk and sugar, then whisk until well combined. Add the eggs and whisk again.
Add the vanilla and mint extracts, baking powder, baking soda and salt, and stir to combine.
Add flour and whisk until smooth and lump-free. It will seem lumpy at first, but keep stirring. It will get nice and smooth. The batter will be thin.
OPTIONAL If making the optional red candy cane swirl (pictured in the post above), remove ½ cup of the batter to a small bowl. Add ½-1 teaspoon of red food color to achieve the hue desired. Stir well. Transfer the remaining batter to the prepared pan. Drop the red batter over the top in spoonfuls. Swirl a few times with a table knife. Don’t swirl too much or the cake will turn pink. Proceed with steps 9 and 10.
If not using the optional swirl technique (step 7 above) transfer all of the batter to the prepared pan.
Bake until golden brown and a toothpick inserted in the center comes out clean, 18-25 minutes.
Cool on a wire rack for 30 minutes before icing: