In your stand mixer with the paddle attachment, beat the softened butter on medium speed until nice and smooth (about 2 minutes).
1 1/2 cups unsalted butter
Reduce to low speed and gradually add the sifted powdered sugar, one cup at a time, incorporating fully between additions.
5 cups powdered sugar
Add vanilla extract and salt, then beat on medium speed for 3-4 minutes until light and fluffy.
2 tsp vanilla extract, 1 pinch salt
Add milk or cream one tablespoon at a time until you reach the desired consistency for frosting and piping.
2-3 tbsp whole milk or heavy cream
Reserve about 1 cup of uncolored buttercream for details.
Add blue gel food colouring to the remaining buttercream and mix thoroughly to create vibrant Bluey blue color. Start with a small amount and gradually add more until you reach the desired shade.
Blue gel food coloring
If necessary, level the tops of your cooled cake layers using a serrated knife to create flat surfaces.
Place the first cake layer on a cake board or serving plate.
Apply a generous layer of blue buttercream on top using an offset spatula.
Place the second layer on top and gently press down to secure.
Apply a thin "crumb coat" of buttercream over the entire cake to seal in any crumbs. Chill in the fridge for 20-30 minutes until set.
Apply the final, thicker layer of blue buttercream to the cake, smoothing the sides and top with your offset spatula and bench scraper to smooth the frosting.
Refrigerate the cake for another 15-20 minutes to firm up before adding the detailed decorations.