Preheat oven to 350 degrees F, and line 2 6-inch round cake pans with parchment paper. Grease the sides. Combine butter, oil, eggs, sugar, milk, and vanilla extract in a mixing bowl and use an electric mixer or a whisk to beat until combined.
Add flour, baking powder, and sea salt. Beat slowly until just combined.
Fold in sprinkles, if using.
Divide into the prepared cake pans. Bake for 42-45 minutes until a toothpick inserted in the center comes out clean.
Cool for 20 minutes, then remove from the pans and transfer to a cooling rack.
Cool completely before frosting. If not frosting right away, you can wrap the cake layers tightly in plastic wrap and store in the fridge.
Using a stand mixer with the paddle attachment or an electric mixer, add all ingredients for the vanilla buttercream frosting.
Whisk on low speed until everything is combined, then switch to the highest speed for 8-10 minutes, stopping to scrape down the sides of the bowl halfway through. The frosting should be smooth and creamy. Add food dye to color the frosting however you like.
If you cake layers are domed on the top, you can use a use a sharp serrated knife to trim the dome to level out the layers. Spread an even layer of frosting in between the two cake layers. Frost the cake however you like!