Step 1 In a medium bowl, stir together the sugar and butter until completely combined and smooth. Stir in the oats until they are coated and the topping forms clumps. Spread the topping onto a rimmed baking sheet and refrigerate until chilled and hardened. Break up the clumps and store, refrigerated, in a sealable plastic bag. This makes slightly more topping than is needed for the muffins; the topping will keep, refrigerated, up to 1 month (or frozen up to 3 months).