In 2-quart saucepan, mix sugar, cocoa, 1/4 cup cornstarch, the salt, milk and egg yolks with whisk until thoroughly combined. Cook over medium-low heat about 15 minutes, beating constantly, until thickened. Once pudding has thickened, use immersion blender, regular blender or food processor to blend in 2 tablespoons butter and the vanilla. Scrape pudding into bowl; cover top with plastic wrap, making sure plastic wrap comes in contact with pudding to prevent a skin from forming. Refrigerate 3 to 4 hours.