1. Combine the sugar, milk, vanilla and butter in the bowl of a stand mixer fitted with a paddle (or a large bowl if using a hand mixer). Beat until smooth, scraping down the sides of the bowl as needed.
2. Swirl in the preserves using a spatula to make pretty streaks.
3. Frost the muffins and watch them turn into cupcakes!
Cook's Note: Frosting can be made 1 day ahead, refrigerate in an airtight container.