Preheat the oven to 350F. Spray a 9" cake pan generously with non-stick cooking spray and set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set this aside.
In a large bowl, whisk the granulated and brown sugar into the melted butter. Make sure to get out any brown sugar lumps, mixture will be gritty. Then whisk in the egg, yogurt, milk and vanilla extract until combined.
Slowly mix in the dry ingredients to the wet, until it's all incorporated and no lumps remain. Slowly fold in the sprinkles, but careful not to overmix, as the sprinkles will begin to bleed their color if you do.
Pour the batter into the prepared cake pan. Bake for about 20 minutes, then remove the pan from the oven and loosely cover it with foil. Return the pan to the oven and continue baking for 13-17 more minutes or until a toothpick inserted into the center comes out clean. Once it's done, allow the cake to cool completely before frosting it.
To make the frosting, beat the butter on medium speed using either a hand held mixer or a stand mixer. Beat until it is smooth and creamy. Add the powdered sugar, cream, vanilla extract and salt with the mixer running. Increase to high speed and beat for 3 or so minutes. Start with 3 cups of the powdered sugar add more, if needed, to thicken the frosting. If the frosting needs to be thinned, then add more cream.
Once the cake has cooled completely, frost the cake and decorate with additional sprinkles, if you'd like. Then you're ready to slice it and serve.
Store the cake covered either in the fridge or at room temperature, for up to 5 days.
