1. Lightly grease a 9×13 pan. Preheat your oven to 325 degrees fahrenheit for a nonstick pan, or 350 degrees fahrenheit for a glass pan. 
2. Melt ¾ cup unsalted butter and let cool.
3. To a stand mixer or bowl, add in your box of white cake mix, ¾ cup milk, ¾ cup melted and cooled butter, 3 whole eggs and 1 egg yolk, and 1 teaspoon vanilla. 
4. Mix at a medium speed 1.5-2 minutes. 
5. Pour batter into your greased pan. 
6. Melt 3 tablespoons butter and let cool. 
7. In the meantime, to a bowl, add ½ cup brown sugar, 1 tablespoon cinnamon, and the 3 tablespoons, melted and cooled butter and mix until combined. 
8. Drop spoonfuls of your sugar mixture to the top of the cake batter.
9. Using a butter knife, swirl the sugar mixture around the cake.
10. Bake for 27-32 minutes until a toothpick comes out clean.
11. Let cool until room temperature before adding the glaze – about 30-45 minutes. 

For the glaze:

1. Whisk together 2 cups powdered sugar, 3 tablespoons whole milk, ½ teaspoon vanilla, and a dash of cinnamon. If it is too thick, add a dash of milk at a time until desired consistency is reached. 
2. Pour the glaze over the cake and spread with a spatula as needed.
3. This glaze will dry fast – top with sprinkles as desired quickly after adding the glaze and enjoy!