Preheat the oven to 350°F. Line a muffin pan with paper cupcake liners.
Make The Streusel:
In a medium bowl, stir together the rolled oats, brown sugar, and whole wheat flour.
Melt the butter in the microwave, then add to the oat mixture and stir together. Set this aside for now.
Make The Muffins:
In a medium bowl mix together the whole wheat flour, all purpose flour, ground flaxseed, baking powder, baking soda, salt and cinnamon. Set aside.
In a large bowl, beat together the softened butter and sugar together with electric mixer.
Add eggs and vanilla extract. Mix together.
Add buttermilk and mix in.
Add half of the dry ingredients to the bowl and mix. Repeat with the other half of dry ingredients.
Batter will be thick and sticky.
Use cookie scoop or spoon to pour batter into each of the muffin cups.
Sprinkle the streusel mixture on top of each muffin.Bake muffins for 22-28 minutes, or until toothpick comes out clean when poked through the center (mine took 27 minutes).
Let cool 10 minutes then transfer to a rack to cool completely.
Recipe Notes:
Store in an airtight container on the counter or in the fridge for up to 5 days. These muffins can also be frozen.