Whisk together the wet ingredients in a bowl.
Fold in the cocoa powder, flour, and baking powder. Stir until almost fully incorporated. The batter will be thick.
Slowly add hot water and fold until smooth.
Pour batter into greased pan. Tap on counter to remove any bubbles. Bake and allow to cool completely.
Make the frosting by beating together SunButter, cocoa powder, and vanilla. Slowly add milk while stirring. Add powdered sugar last.
Assemble the cake by using a spoon to place frosting onto cake. Use a spatula or knife to spread.