• Preheat oven to 425ºF. Grease a 2 (12 cup), standard size muffin tin with non-stick cooking spray or line with liners. You'll only use 18 slots.

Filling

• In a small bowl stir together butter, brown sugar, and cinnamon until smooth.
• Set aside.

Streusel

• In a medium sized bowl, stir together flour, oats, salt, cinnamon, and brown sugar.
• Using a fork, cut in butter until the mixture resembles coarse crumbles. Set aside.

Muffin

• In a large bowl, whisk together flour, cornstarch, baking powder/soda, salt, cinnamon, sugar, and brown sugar until well combined. Set aside.
• In a medium sized bowl, whisk together butter, buttermilk, eggs, and vanilla.
• Using a rubber spatula, stir together wet and dry ingredients.
• DON'T OVERMIX.
• In your prepared pans, place 2 tablespoons of batter in the bottom of each cup.
• Top with cinnamon filling (about 1 teaspoon per slot).
• Top with remaining batter (about 1 tablespoon per slot).
• Top with streusel. It's ok if you don't use all the streusel.
• Bake in preheated oven for 5 minutes.
• After 5 minutes, lower oven temperature to 375 degrees F and bake for an additional 8 minutes.
• Remove from oven and let cool for a few minutes before removing from the tin and serving.
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