Preheat your oven to 350°F (180°C) and line a 24-hole mini cupcake tin with paper liners. In the bowl of a stand mixer fitted with the paddle attachment, combine the self-raising flour, all-purpose flour, and sugar. Mix on low speed for about two minutes. Gradually add the cubed butter, one piece at a time, allowing each to incorporate before adding the next. Continue mixing until the mixture resembles fine sand. Pour in half the milk along with the vanilla extract and mix until just combined, about one minute. Add the eggs one at a time, scraping down the sides of the bowl after each addition. Pour in the remaining milk and mix on low speed for another 2–3 minutes until the batter is smooth and fully blended. Spoon roughly 1 tablespoon of batter into each cupcake liner. Bake for 14–16 minutes, or until the tops spring back when gently pressed. Remove from the oven and cool completely on a wire rack before frosting.

Place the softened butter into the bowl of a stand mixer fitted with the paddle attachment. Beat on high speed until the butter is pale and fluffy. Add the sifted powdered sugar and vanilla extract, then continue to beat on high until the mixture is fully combined and smooth. Gradually add the milk, one tablespoon at a time, beating after each addition, until the buttercream reaches a smooth and spreadable consistency. Divide the buttercream evenly between three bowls and tint each one with a different gel food coloring to create your desired galaxy color palette.

Add the softened butter to the bowl of a stand mixer fitted with the paddle attachment. Beat on high speed until light and fluffy. Add the sifted powdered sugar, black cocoa powder, and vanilla extract, and continue to beat on high until thoroughly incorporated. Slowly add milk, one tablespoon at a time, beating between additions, until the buttercream is smooth and easy to pipe.

Fit a large piping bag with a closed star tip (such as a Wilton 1M or 2D). Using a tablespoon, alternate spoonfuls of the colored buttercreams into the piping bag, layering the colors to create a swirling galaxy effect. Once the piping bag is filled, pipe a generous swirl of buttercream onto each cooled cupcake. To create a starry galaxy look, place a small amount of gel food coloring into a bowl and dilute with a few drops of water if needed. Dip your finger or a clean brush into the coloring and flick small specks over the frosted cupcakes to resemble stars. Finish with a light dusting of edible glitter on each cupcake for a celestial sparkle.