In a bowl, add the gluten free flour blend, sugar, baking powder, and salt and whisk to blend.
In another bowl, add the smooshed banana, butter, egg, vanilla, and milk. Whisk to blend.
Pour the wet ingredients into the dry ingredients and mix until just barely mixed.
Put the cake batter into a microwave-safe mug and microwave for 90 seconds.
Check the top of the cake with your finger. If it is a bit sticky, microwave an additional 35 seconds and re-test. The exact time will vary depending on how tall and wide your mug is.
Dump the cake out onto a plate for cooling.
Allow to cool and top with frosting, ice cream, or whipped cream