start by creaming together your softened butter till you get a lighter silky texture (about 3-5 min)

now add your shortening and whip for about 1 minute or so on high speed

now slowly start adding your salt and 1 cup at a time of powdered sugar, stopping your mixer ever minute or so to scrape sides continue to add your powder sugar.

once it thickens start added your heavy cream, extract and the last powder sugar and cornstarch till a nice consistency.

your buttercream should stick to the spatula if its still to loose add 1/2 cup more powder sugar, and tspn at a time of cornstarch.

I like to set in fridgerator overnight covered in a Tupperware container for best results. enjoy!