Start by melting the butter in the mug, Add the peanut butter, and stir until combined. Add in the egg yolk, the brown sugar, salt, the milk, and the vanilla and stir again.
1 & 1/2 tbsp butter, 1 & 1/2 tbsp peanut butter, 1 egg yolk, 2 tbsp brown sugar, 1 & 1/2 tbsp milk, 1/8 tsp vanilla extract, a pinch of salt