Melt your butter in a small saucepan at high heat, making sure to swirl it constantly until the foaming stops.
Slide that butter into a 1 cup liquid measuring cup.
Placing the head of your hand blender at the bottom of the plastic cup, mix the egg yolk, water, lemon juice and a pinch of salt.
While the hand blender is mixing the ingredients in the cup, slowly pour the hot butter into the cup. Keep that up until all the butter is gone.
Season to taste with kosher salt, and if you’re feeling like you want that extra something, add cayenne pepper or hot sauce.
Serve it immediately.