• Preheat the oven to 375℉/190℃. 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, oats, and butter. Using your hands or a fork, mix 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 into the fridge 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 the dry ingredients. Using a spatula, mix and set aside.
  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 the wet ingredients. Whisk for 2-3 minutes.
  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 with 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 and add a handful of streusel on top of each muffin.
• Bake the muffins for 23-27 minutes. The muffins are ready when a toothpick inserted comes out clean.