Making the Cake:

1. Prepare your oven and pans: Preheat oven to 350°F (175°C). Grease three 8-inch or 9-inch round cake pans with butter or cooking spray, then line the bottoms with parchment paper circles. Grease the parchment too for easy release.
2. Mix dry ingredients: In a large bowl, whisk together the flour, baking powder, and salt. Set aside while you prepare the wet ingredients.
3. Cream butter and sugar: In your mixer bowl, beat the softened butter and sugar on medium-high speed for 4-5 minutes until light, fluffy, and pale in color. This step incorporates air for a lighter cake, so don’t rush it.
4. Add eggs: Add eggs one at a time, beating well after each addition. The mixture might look slightly curdled at first—this is normal. Add the vanilla extract and mix until combined.
5. Alternate dry and wet ingredients: With mixer on low speed, add the flour mixture in three parts, alternating with the milk in two parts. Start and end with flour (flour-milk-flour-milk-flour). Mix just until you can’t see dry streaks—overmixing creates tough cake.
6. Divide and bake: Divide batter evenly between your three prepared pans (about 2 cups per pan for 8-inch pans). Smooth the tops with a spatula. Bake for 28-32 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs. The tops should spring back when lightly touched.
7. Cool completely: Let cakes cool in pans for 15 minutes, then turn out onto wire racks. Remove parchment paper and let cool completely before decorating—at least 1 hour. Warm cake will melt your buttercream!

Making the Galaxy Buttercream:

8. Beat the butter: In a large, clean mixing bowl, beat the softened butter on medium speed for 3-4 minutes until smooth and creamy.
9. Add sugar gradually: With mixer on low, gradually add the powdered sugar, one cup at a time. Once all the sugar is incorporated, increase speed to medium-high and beat for 3 minutes.
10. Add cream and vanilla: Add the heavy cream and vanilla extract. Beat on high speed for 4-5 minutes until the buttercream is light, fluffy, and smooth. It should be spreadable but hold its shape.

Assembling and Decorating:

11. Level your cakes: If the cake layers domed during baking, use a cake leveler or serrated knife to trim them flat. This ensures stable, even layers.
12. Create base layer: Place your first cake layer on a cake board or serving plate. Spread about ¾ cup of plain white buttercream on top, going almost to the edges. Add second layer and repeat. Top with third layer, bottom-side up for a flat top surface.
13. Apply crumb coat: Spread a thin layer of white buttercream all over the entire cake—top and sides. This is your crumb coat that traps loose crumbs. It doesn’t need to look perfect. Refrigerate for 20-30 minutes until firm.
14. Divide and color buttercream: Divide remaining buttercream into 4-5 bowls. Leave one bowl white (or light purple). Color others with gel food coloring: one deep purple, one royal blue, one hot pink, and optionally one black or very dark purple. Make colors vibrant and saturated.
15. Apply galaxy effect: Using an offset spatula or butter knife, apply large dollops of your colored buttercream randomly over the entire cake. Don’t overthink placement—space is naturally chaotic! Start with larger areas of purple and blue, then add pink accents.
16. Blend the colors: Using a clean offset spatula or palette knife, gently blend where colors meet, creating swirls and gradients. Use light strokes and don’t overblend—you want distinct color areas that flow into each other, not one muddy color. Rotate the cake stand (or walk around your cake) as you work.
17. Add depth with black: Take tiny amounts of black buttercream and swirl it into the darker areas for depth. Less is more here—just a few strategic touches.
18. Smooth technique: For a smoother finish, dip your spatula in hot water, wipe it dry, and gently smooth areas while maintaining the color variation.
19. Add the stars: While buttercream is still soft, scatter edible star sprinkles across the cake, concentrating them in certain areas like real star clusters. You can also use a small brush to apply silver or pearl luster dust for a shimmery effect.
20. Final touches: For extra magic, dust the entire cake lightly with edible glitter. Step back and admire your cosmic creation! Refrigerate for 30 minutes to set before slicing.