Preheat oven to 325°F and line 3 round cake pans (8 or 9") with parchment paper rounds. Spray the edges of the pan with cooking spray.
In the bowl of a stand mixer, add the cake mix, flour and sugar and mix.
Add the sour cream, butter, milk, eggs, extract and sprinkles. Beat for 1 minute, scrapping the sides down half-way through.
Divide the batter evenly between the three prepared pans (about 2 cups of batter each).***
Bake for 20-25 minutes or until a toothpick poked into the middle of the cake comes out clean.
Let the cakes cool for a few minutes, then go around the edges with a butter knife to help release the cake. Then, flip the pans over to release the cake onto a cooling rack. Remove the parchment paper and let cool completely.
When the cakes are cool, it's time to make the buttercream and assemble. To make the buttercream, place the butter in the bowl of a stand mixer and beat until smooth.
Add powdered sugar, vanilla and 1 tbsp milk and mix slowly until combined.
Add additional milk if needed, mixing well between each addition.
Place one cake layer on the bottom of a platter. Spread frosting over the cake in a smooth, even layer.
Repeat until you have all three layers stacked.
Frost the edges and top of the cake, making a thin layer of frosting called the crumb coat (don't worry if there are some crumbs in there - it'll be covered later). Place in the freezer for 30 minutes.
Once the crumb coat is firm, you'll put the final coat of buttercream on!
Top with sprinkles, slice and enjoy!