• Preheat the oven to 350°F. Line a muffin pan with paper cupcake liners.

Make The Streusel:

• Mix ¼ cup old-fashioned rolled oats, ¼ cup brown sugar, and 2 Tbsp whole wheat flour in a medium bowl.
• Melt 3 Tbsp unsalted butter in the microwave, then add to the oat mixture and stir. Set this aside for now.

Make The Muffins:

• Mix your dry ingredients in a medium bowl. 1 cup whole wheat flour, ¾ cup all-purpose flour, ¼ cup ground flaxseed, 1 tsp baking powder, 1 tsp baking soda, 1 tsp cinnamon, and ¼ tsp salt. Set aside.
• In a separate large bowl begin adding the wet ingredients: ½ cup unsalted butter, and 1 cup brown sugar. Mix with an electric mixer.
• Add 2 eggs and 1 tsp vanilla extract to the wet ingredients, and mix.
• Mix 1 ½ cups buttermilk to the wet ingredients.
• Mix half the dry ingredients into the wet ingredients, then repeat with the second half. Batter will be thick and sticky.
• Pour batter into each muffin cup using a cookie scoop or spoon.
• Sprinkle the streusel mixture on top of each muffin. Bake muffins for 22-28 minutes, or until a toothpick comes out clean when poked through the center (mine took 27 minutes).
• Let cool for 10 minutes then transfer to a rack to cool completely.