Beat the egg whites, salt and cream of tartar, slowly add in 1/3 cup of sugar and continue beating until soft peaks form.
In a medium saucepan add the remaining sugar and 1/3 cup water then place on medium-low heat.
Stir until sugar melts and becomes clear.
Maintain at medium-high heat until temperature reads 235-240F.
Drizzle the sugar into the mixer immediately.
Run the mixer until meringue is cool/tepid, about 15 minutes. You can pack the bowl with ice or frozen peas if you want to speed the cooling down.
Switch to a paddle attachment. Add room temperature butter into running mixer one tablespoon piece at a time.
Add the salt and vanilla.
Beat until butter is combined and mixture has reached a silky consistency.
Distribute into multiple bowls.
Use food coloring to create a gradient of colors.
Transfer to piping bags. Snip off the tips of each bag.