Place the butter in a 8 x 12 x 2-in sheet pan. Preheat the oven to 325ºF and while it reaches temperature place the pan inside for the butter to melt.
Meanwhile, whisk together the flour, baking powder, sugar and cocoa together in a mixing bowl.
When the butter has melted, pour into a jug and set aside to cool for a few minutes. Meanwhile, brush the pan with the residual butter so all the sides are coated.
Add the milk and eggs to the melted and cooled butter and mix well. Stir this into the dry ingredients to make a smooth batter. Scrape into the buttered sheet pan and smooth the top with a spatula.
https://www.thedailymeal.com/recipes/magic-melting-mocha-cake-recipe