In the top half of a double boiler set over simmering water, combine sugar, cream of tartar, salt, water, and egg white. (Do not let bottom of bowl touch water.) Beat at high speed with a mixer until frosting thickens and forms stiff peaks, 7 to 10 minutes.
Use immediately.
Recipe by TeaTime Magazine at https://teatimemagazine.com/pineapple-filled-white-layer-cake-recipe/