Put the butter into a microwave safe mug and microwave for 20 seconds to melt.
Add the egg yolk, milk, vanilla and sugar, then stir with a fork to combine. Add the flour and stir vigorously until batter is smooth. Microwave for 90 seconds to 2 minutes, until the cake has risen and top of the cake is firm to the touch.
Let cool for 2 minutes before decorating.
In the meantime whisk together the confectioners’ sugar with the milk until smooth. Use food coloring to dye glaze pink.
When cake is cool, using a melon baller, scoop out a deep divot into the center of the cake. Top with glaze spreading to coat evenly allowing a bit to fall into the center and drip down the sides.