Make the buttercream: beat butter until smooth in the bowl of an electric mixer. Slowly add powdered sugar, 1 cup at a time, until crumbly. Add vanilla, salt, and 1-2 tablespoons of heavy whipping cream, then mix until smooth and creamy. Add more heavy whipping cream as needed (it kind of depends on how soft your butter is, the frosting needs to be spreadable).
If you haven't filled your cake already: level the two cakes. Place one layer on cake plate, fill with frosting (or use a favorite filling, like lemon curd). Then frost the outside of the cake with a thin layer of frosting. This is your "crumb coat". Chill until set, about 30 minutes to one hour.
When ready to do your decoration: tint your frosting the desired color yellow. Fill a pastry bag fitted with a 1M (large star) tip.
Slice the top off the pineapple and level it as best as possible. Take a small amount of plastic wrap and wrap the bottom of the topper, so it doesn't get the cake soggy. Place it in the center of the cake.
Pipe rose swirls (us a simple swirl motion) to create the decoration all over the cake. Because it's hard for me to be perfect, I tend to not go in rows; instead I alternate and then fill in any holes with smaller rosettes or a simple squeeze of icing.
Place white pearl sprinkles around the cake in the center of the roses.
Chill cake to set.
Optional: Have a emoji loving kid? Skip the pearls and use black icing to pipe eyes and a smile for a pineapple emoji!