Step 1: Preheat and Prepare Pans
Preheat your oven to 350°F (175°C). Grease two or three 8-inch (or 9-inch) round cake pans with butter or baking spray, then line the bottoms with parchment paper rounds. Grease the parchment too for insurance. This double-layer of protection ensures your cakes release perfectly every time.

Step 2: Mix Dry Ingredients
In a large bowl, whisk together the flour, baking powder, baking soda, and salt until well combined. Whisking ensures all leavening agents are evenly distributed for uniform rising. Set this bowl aside.

Step 3: Cream Butter and Sugar
In your stand mixer bowl (or large bowl with hand mixer), beat the room-temperature butter on medium speed for about 1 minute until smooth. Add the sugar and beat on medium-high speed for 3-4 minutes until light, fluffy, and pale in color. This creaming process is crucial—it incorporates air that creates light, tender cake texture. Scrape down the bowl sides occasionally.

Step 4: Add Eggs and Vanilla
Add the eggs one at a time, beating well after each addition (about 30 seconds per egg). The mixture may look slightly curdled—that’s normal! Add the vanilla extract and beat until combined. Scrape down the bowl to ensure everything is evenly mixed.

Step 5: Combine Wet Ingredients
In a separate measuring cup or bowl, whisk together the milk, sour cream, and vegetable oil until smooth. Having these combined makes it easier to add them to the batter alternately with the dry ingredients.

Step 6: Alternate Adding Dry and Wet Ingredients
With your mixer on low speed, add the flour mixture in three additions, alternating with the milk mixture in two additions. Start and end with flour: flour, milk mixture, flour, milk mixture, flour. Mix just until each addition is incorporated—don’t overmix! Scrape down the bowl sides and bottom to catch any unmixed bits.

Step 7: Fold in Sprinkles
Using a rubber spatula, gently fold the rainbow sprinkles into the batter by hand. Don’t use the mixer or the sprinkles might bleed color. Fold just until the sprinkles are evenly distributed throughout the batter. The batter should be thick, smooth, and gorgeously speckled with color!

Step 8: Divide and Bake
Divide the batter evenly among your prepared pans (use a kitchen scale for precision—each pan should have equal weight). Smooth the tops with a spatula. Bake for 28-32 minutes (check at 28 minutes), or until a toothpick inserted in the center comes out clean or with just a few moist crumbs, and the tops spring back when lightly touched. The cakes should be golden and pulling away slightly from the pan sides.

Step 9: Cool Completely
Let cakes cool in the pans on wire racks for 10-15 minutes. This brief resting period helps them set without becoming soggy. Then carefully turn them out onto the wire racks, remove parchment paper, and let them cool completely (at least 1 hour) before frosting. Frosting warm cakes will cause the frosting to melt!

Step 10: Make the Buttercream Frosting
In a clean mixing bowl, beat the room-temperature butter on medium speed for 2 minutes until creamy and smooth. Add 4 cups of powdered sugar, vanilla, and salt. Beat on low speed until the sugar is incorporated, then increase to medium-high and beat for 2-3 minutes until fluffy. Add heavy cream 1 tablespoon at a time until you reach your desired consistency. Add more powdered sugar if too thin, more cream if too thick.

Step 11: Level and Layer
If your cake layers have domed tops, use a serrated knife to carefully level them so they stack evenly. Place your first layer on a cake stand or serving plate, spread about 1 cup of frosting evenly on top, place the second layer, and repeat. Add the final layer and frost the top and sides of the entire cake. Use an offset spatula for smooth, professional results.

Step 12: Decorate and Serve
Decorate the frosted cake with rainbow sprinkles—press them gently into the sides and sprinkle generously on top. Get creative! Refrigerate the cake for 30 minutes to set the frosting before slicing for the cleanest cuts. Slice, serve, and watch everyone’s faces light up!