• Preheat the oven to 425℉/218℃. Line your muffin tin with 12 liners and set aside.

For the streusel

• In a medium-sized bowl, add flour, white sugar, dark brown sugar, cinnamon, rolled oats, and melted and cooled (but still liquid) butter. Using your hands, mix this until crumbly and combined. You may need to sprinkle an extra 1-2 tbsp of flour on top and continue to mix together if your butter isn't completely absorbed. Set aside while you make the muffin batter.
  1/2 cup all-purpose flour, 1/4 cup white sugar, 3 tbsp dark brown sugar, 1/3 cup rolled oats, 1/4 tsp cinnamon, 4 tbsp unsalted butter

For the muffins

• In a medium-sized bowl, add in flour, oats, cinnamon, salt, cornstarch, baking powder, and baking soda. Using a spatula, mix until just completely combined.
  1 1/2 cups rolled oats, 1 3/4 cups all-purpose flour, 1 tsp cinnamon, 1 tbsp cornstarch, 1 1/2 tsp baking powder, 1 tsp baking soda
• In a large bowl add eggs, milk, vanilla, brown sugar, oil, and sour cream. Whisk until combined.
  1/2 cup milk, 3/4 cups full-fat sour cream, 2/3 cups oil, 3/4 cups light brown sugar, 2 eggs, 1 tbsp vanilla
• Add the dry into the wet and using a spatula, gently fold until a soft batter forms.
• Mix in the mini chocolate chips until just combined. Do not over-mix!
  3/4 cups mini chocolate chips
• Evenly divide the batter between 12 liners. Using a cookie scooper helps big time to minimize the mess. Sprinkle a handful of streusel on top of each muffin.
• Bake the muffins at 425℉/218℃ for 5 minutes and then 375°F for 14-18 minutes. The muffins are ready when a toothpick inserted comes out clean.
• These muffins stay fresh covered at room temperature for up to 3 days, and in an airtight container in the refrigerator for up to a week!