1. Preheat and prepare pans: Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans with butter or baking spray, line the bottoms with parchment paper circles, then grease the parchment. Dust lightly with flour and tap out excess. This triple protection ensures easy release!
2. Mix dry ingredients: In a large bowl, whisk together flour, baking powder, baking soda, and salt. Set aside. Whisking aerates the flour and ensures even distribution of leavening agents.
3. Cream butter and sugar: In a stand mixer (or large bowl with hand mixer), beat the room-temperature butter on medium speed for 2 minutes until smooth and creamy. Add sugar and beat on medium-high speed for 4-5 minutes until light, fluffy, and pale in color. This creaming process incorporates air, creating a lighter cake. Stop and scrape down the bowl as needed.
4. Add eggs: Add eggs one at a time, beating well after each addition (about 30 seconds per egg). Make sure each egg is fully incorporated before adding the next. The mixture may look slightly curdled—this is normal. Add vanilla extract and mix until combined.
5. Alternate dry and wet ingredients: With mixer on low speed, add the flour mixture in three additions, alternating with the milk in two additions, beginning and ending with flour. Mix each addition just until combined—about 10-15 seconds. The pattern is: ⅓ flour, ½ milk, ⅓ flour, ½ milk, ⅓ flour. Don’t overmix!
6. Finish mixing: Stop the mixer and use a rubber spatula to scrape the bottom and sides of the bowl, then gently fold a few times to ensure everything is incorporated. The batter should be smooth and fluffy.
7. Divide and bake: Divide batter evenly between the two prepared pans (about 2½ cups per pan—you can weigh them for precision). Spread evenly and tap pans gently on the counter 2-3 times to release air bubbles. Bake for 28-32 minutes, 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.
8. Cool completely: Remove from oven and let cakes cool in pans on wire racks for 10 minutes. Run a knife around the edges, then carefully invert onto racks. Remove parchment paper and let cool completely—at least 1-2 hours. Cakes must be completely cool before frosting or the buttercream will melt!

Make the Buttercream Frosting:

9. Beat the butter: In a stand mixer (or large bowl with hand mixer), beat room-temperature butter on medium speed for 3-4 minutes until smooth, creamy, and pale. Properly beaten butter is the key to smooth buttercream.
10. Add sugar gradually: With mixer on low speed, add powdered sugar 1 cup at a time, beating after each addition. Start on low to prevent a sugar cloud! Once all sugar is incorporated, increase speed to medium-high and beat for 2 minutes.
11. Add liquid and flavorings: Add heavy cream (or milk), vanilla extract, and salt. Beat on medium speed for 30 seconds, then increase to medium-high and beat for 3-4 minutes until light, fluffy, and smooth. If too thick, add more cream 1 tablespoon at a time. If too thin, add more powdered sugar ¼ cup at a time.
12. Add color if desired: If you want colored frosting, add gel food coloring a tiny bit at a time, beating until evenly distributed.

Assemble the Cake:

13. Level the layers (optional): If your cake layers domed during baking, use a serrated knife or cake leveler to trim the tops flat for even stacking.
14. First layer: Place one cake layer on your serving plate or cake board, cut side up if leveled. Place strips of parchment paper under the edges to keep the plate clean while frosting (remove later).
15. Add filling: Spread about 1 cup of buttercream evenly over the first layer, going almost to the edges (it will spread when you add the top layer).
16. Stack second layer: Place the second cake layer on top, cut side down if leveled. Press down gently to adhere.
17. Crumb coat: Apply a very thin layer of frosting all over the cake (top and sides). This “crumb coat” seals in crumbs. Refrigerate for 15-20 minutes to set the crumb coat.
18. Final frosting: Apply remaining frosting generously over the chilled crumb coat, smoothing the top and sides with an offset spatula or bench scraper. Use a cake turntable if you have one for easier frosting.
19. Decorate: Add sprinkles, piped decorations, or keep it simple—the choice is yours! Remove parchment strips carefully.
20. Serve: Let the cake sit at room temperature for 30 minutes before slicing for the best texture. Use a sharp knife dipped in hot water and wiped clean between cuts for neat slices.