Preheat oven to 350 degrees Fahrenheit and generously grease a 9-inch round cake pan.
In a large mixing bowl, combine cake mix, butter, eggs, milk, and vanilla extract. Mix well until all the ingredients are well incorporated.
Gently stir in the sprinkles, making sure they are evenly distributed throughout the batter.
Pour the prepared batter into the greased cake pan, using a spatula to smooth the top surface.
Tap the cake pan lightly on the counter to release any trapped air bubbles.
Place the cake pan in the preheated oven and bake for 35-40 minutes, or until a toothpick inserted in the center comes out clean.
Once the cake is completely baked, remove it from the oven and transfer it to a wire rack to cool completely.
While the cake is cooling, prepare the vanilla buttercream frosting. In a separate mixing bowl, combine 1 cup unsalted butter, 4 cups powdered sugar, 1 tablespoon vanilla extract, and 2-3 tablespoons milk.
Using an electric mixer, beat the frosting ingredients together until smooth and creamy.
Once the cake is completely cooled, frost the top and sides of the cake with the prepared vanilla buttercream frosting.
Your cake is now ready to serve and enjoy!