If you’re only going to be icing the cake and piping simple decorations, then Easy Buttercream will work just fine. Since it’s made with butter, it will soften at room temperature and have a slightly yellow tint to it.
1/2 cup butter (1 stick), softened
1/2 cup solid vegetable shortening
1 pound powered sugar
2 tablespoons milk (or water)
1 teaspoon pure vanilla extract
Using an electric mixer, cream shortening and butter. Add vanilla, sugar, and milk/water. Beat on medium speed until light and fluffy, scraping down bowl as needed.
To make a simple Chocolate Buttercream, add 3/4 cup cocoa powder (or 3 ounces of melted and cooled bittersweet chocolate) and an additional 1-2 tablespoons milk to the buttercream icing above.