Preheat the oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment paper.
Whisk together the flour, baking powder, and salt.
In another bowl, beat the softened butter and sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Mix in the vanilla.
Add the dry ingredients alternately with the milk, mixing just until combined. Do not overmix.
Divide the batter evenly between the prepared pans.
Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean.
Cool the cakes in the pans for 10 minutes, then transfer to a wire rack and cool completely.
For the buttercream, beat the softened butter until smooth and creamy.
Gradually add the powdered sugar, then mix in the vanilla.
Add the cream or milk gradually until the frosting reaches a smooth, spreadable consistency. Beat until fluffy.
Tint the main portion of buttercream golden yellow. Reserve smaller portions for the lion's mane and facial details.
Place one cake layer on a serving plate and spread buttercream over the top.
Add the second cake layer and apply a thin crumb coat. Chill for about 15 minutes.
Cover the cake with the remaining golden buttercream and smooth the sides.
Pipe a lion face and mane onto the cake using the colored buttercream, or decorate with a Leo zodiac symbol, stars, and gold accents.
Chill briefly to set the decoration before serving.