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Here is a topping I use on my Streusal Apple Pie. Sounds like what you are looking for. Will work on muffins too. It will give you that crunch topping like the muffins in supermarkets.
1/2 cup softened Margarine
1/2 cup sugar
1 1/2 cups flour
Whisk the crumb mixture together and pat on muffins. Bake same as the muffin recipe.
Also Recipezaar has a recipe called Muffin Toppings #90227. It has three toppings. Nut Topping; Streusal Topping; and a Sugar & Cinnamon Topping.
By Chris T. (Guest Post)01/12/2009
I used to bake a lot. For really crunchy tops, I assume you mean the ones you cook yourself. You could sprinkle the top of the muffin, before baking with just plain granulated sugar, not a lot, but just some, or sprinkle the top with powered sugar. I use to use it on the tops of brownies, before baking & it worked really well. Also on other things, I used to beat up a little egg white & brush the tops of things, before baking. I also use to buy the ones donuts shops sell & I would heat it in the microwave & pop it in a really hot oven for about 10 mins. That works great for the ones you buy, because after they sit awhile, they are no longer crunchy.
By KL 01/07/2009
There are different types of decorating sugars that are coarser than regular granualated sugar we use here for baking with. Sanding sugar or decorating sugar or sparkling sugars don't dissolve under the heat of baking so it does give more of a crunch. We don't have a Dunkin Donuts around here but that is what our bakery uses on muffins and some pastries.
There are many different kinds of sugar listed at King Arthur's site you can read about:
By amie (Guest Post)01/07/2009
I'm not sure if this is the kind your talking about but I make a crumble mixture with butter and brown sugar then put it on top of the muffin before baking.
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