Place butter in a large mug and heat in the microwave until melted, about 40 seconds. (This greases the mug.)
In a small bowl, whisk together flour blend, sugar and baking powder.
In a separate bowl, whisk together egg yolk, milk and vanilla bean paste.
Stir wet ingredients into dry mixture. Add melted butter, stirring until ingredients are combined and batter is smooth.
Pour batter into greased mug and smooth the top with a rubber spatula.
Place mug in the microwave and cook on high 60 to 80 seconds (more if necessary). Cake is done when the top no longer appears wet and the cake pulls away from the sides.
Notes
For Egg-Free Vanilla Minute Mug Cake, omit 1 egg yolk. Add 1 tablespoon arrowroot powder or cornstarch to dry ingredients. Combine 2 teaspoons golden flax meal with 4 to 5 teaspoons hot water or hot milk of choice. Add this mixture to milk and vanilla in step 3.TIP: Microwaves vary, so watch closely to make sure your mug cake doesn’t burn. If it’s tough and dark brown on the bottom when it’s removed, it’s overcooked.