1. Preheat the oven to 350 F and spray a 12 x 17″ sheet pan with nonstick spray, then line it with enough parchment paper that the sides overhang. In a large mixing bowl, whisk together the flour, baking powder, baking soda and salt.
2. In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld electric mixer, beat the softened butter on high for 1-2 minutes, before adding in the granulated sugar. Beat the butter and sugar together on high for another 1-2 minutes.
3. With the mixer running on low, add in the eggs, one at a time, followed by the vanilla extract, then the sour cream. Mix on medium until everything looks combined.
4. With the mixer running on low, add in half of the dry ingredients, followed by half of the buttermilk. Add the remaining dry ingredients, followed by the remaining buttermilk. Mix on low until thoroughly combined, but don’t overmix!
5. Using a spatula, fold in the sprinkles. Empty the cake batter out on to the prepared sheet pan, then smooth it out with a spatula. Bake the cake for 20-25 minutes or until a toothpick inserted in the middle comes out clean.
6. Cool the cake for 15-20 minutes before running a knife gently along the edges of the cake, then carefully grabbing the edges of the parchment paper and lifting the cake out of the pan and on to a large cutting board or wire rack.
7. Make the buttercream. In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld electric mixer, mix the unsalted butter on high until smooth and creamy, about 2-4 minutes.
8. Add in 1 cup of the powdered sugar, along with the vanilla extract. Keep adding in the powdered sugar, one cup at a time, along with the heavy cream, mixing on high until smooth and creamy! Add either more powdered sugar or slightly more cream, until the frosting is smooth. Fold in the sprinkles with a spatula!
9. Make the cake sandwiches. Once removed from the cake pan, carefully cut the cake horizontally, in half. Spread all of the frosting over one half of the cake, smearing it evenly.
10. Working carefully, place both hands under the parchment paper on the side of the cake with the frosting on it, and in one smooth motion, flip the cake over on to the unfrosted side.
11. Make another cut horizontally through both cake halves, then one more cut vertically. Next, make 2 diagonal cuts through the cake halves to form 8 pieces/sandwiches.
12. Use a spatula to scoop up and plate the individual cake sandwiches. You can also make additional cuts to the cake to make smaller pieces! Serve with a fork or just use your hands like a regular sandwich!

Cake sandwiches will keep fresh, at room temperature, for 2-3 days in an airtight container. You can also freeze the cake for up to 2 months.