Place a rack on the lower third position in the oven and begin preheating to 350° F (176° C). Line or grease a 12 well muffin pan.
Melt the butter in the microwave, about 30-45 seconds, and heat up the water, about 30 seconds.
In a mixer bowl or large bowl if mixing by hand, add the dry ingredients and stir to combine or fluff with a fork.
Add the eggs and vanilla then beat together for about 1 minute on medium speed.
Pour in half of the water and beat for about 20-30 seconds then scrape down the bowl. Add the rest of the water and beat until smooth or about another 20-20 seconds.
Split the batter evenly between the twelve wells and bake for 18-22 minutes, spinning the pan about halfway through baking so back is front, front is back. Check for doneness with a toothpick; a few crumbs attached but clean is perfect.
Cool in the pan for about 5 minutes then remove the cupcakes to a cooling rack. They are ready to frost when cooled completely.
Though untested here, you can likely swap in GF flour 1 for 1 for a gluten free cupcake.