In a large bowl, mix together flour, both sugars, baking powder, baking soda, and salt. Set aside.
In a large microwave-safe bowl, melt butter in the microwave.
In a separate jug, whisk the egg, buttermilk, and vanilla extract until combined.
Stir in buttermilk mix into the dry ingredients, followed by the butter until no lumps remain. The batter will be thick.
Lastly, stir in sprinkles, but do not overmix because the sprinkles will bleed their color.
Divide the batter evenly between 3 silicon microwave safe cake tins.
One at a time microwave each cake for 2 minutes and 30 seconds. You will know when the cake is done when it's firm to the touch in the middle and it pulls away from the sides of the pan.
Remove from the microwave and allow to cool before turning out. (this cook time is based on my microwave which is 1200Watts so your cook time may vary)
Frost cooled cake: Level the tops of the cakes if they are lumpy.
Start by frosting a thin ‘crumb layer’ with my Best Ever Buttercream Frosting recipe. Chill for 30 minutes to set the frosting.
Once that layer is set apply a second layer of thicker frosting
Decorate as desired with sprinkles. There may be leftover frosting depending how much you wish to use.
Cover cake and store at room temperature for up to 7 days. Enjoy!