Helen's Soft Chocolate Frosting

Combine white sugar, brown sugar, cocoa, and cornstarch in a medium saucepan, whisking until no lumps remain.

Gradually add milk while stirring constantly to create a smooth paste without lumps.

Place the saucepan over medium heat and cook, stirring frequently, until the mixture thickens and comes to a gentle boil.

Remove from heat and stir in the margarine until completely melted and incorporated into the frosting.

Allow the frosting to cool to room temperature, stirring occasionally, until it reaches a spreadable consistency before using on cakes or cupcakes.