Prep: Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, and line the bottoms with parchment paper.
Whisk Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt.
Beat Wet Ingredients: In a large bowl with an electric mixer, beat the room-temperature butter and vegetable oil until smooth. Add the eggs one at a time, beating well after each. Beat in the vanilla extract.
Combine: Add the dry ingredients to the wet ingredients in three additions, alternating with the milk (Dry, Milk, Dry, Milk, Dry). Mix on low speed only until just combined. Do not overmix.
Fold in Sprinkles: Using a rubber spatula, gently fold the 3/4 cup of rainbow jimmies into the batter. Be gentle to avoid color bleeding.
Bake: Divide the batter evenly between the two prepared pans. Bake for 30-35 minutes, or until the centers are set and a wooden skewer inserted into the center comes out clean.
Cool: Let the cakes cool in the pans on a wire rack for 10-15 minutes. Then, invert the cakes onto the wire rack, remove the parchment paper, and let them cool completely.
Frost: Once completely cool, frost the cake layers with vanilla buttercream. Decorate the top and sides with additional sprinkles.