Preheat oven to 350°F (177°C). Spray a 9-inch springform or baking pan (round or square) generously with nonstick spray. Set aside.
In a medium bowl, mix together flour, baking powder, baking soda, and salt. Set aside.
In a mixing bowl pour the melted butter. Whisk in the sugars vigorously getting out any brown sugar lumps – mixture will be gritty. Whisk in egg, yogurt, milk, and vanilla extract until combined.
Slowly mix in dry ingredients until no lumps remain. Batter will be thick. Slowly stir in sprinkles, but do not overmix because the sprinkles will bleed their color.
Pour/spoon batter into prepared cake pan. This cake takes around 33-37 minutes to bake. What I suggest is to bake it for 20 minutes, then cover loosely with aluminum foil. Continue baking for 13-15 more minutes or until a toothpick inserted in the center comes out clean. Allow to cool.
Melt the butter and condensed milk together. Transfer this to a mixing bowl,
Add icing sugar little by little. Mix it well until you get the thick mixture. Pour over the cake. Garnish with sprinkle.