Preheat oven to 350°F. Prepare muffin tin with liners. Combine ground oats, rolled oats, baking powder, baking soda, salt and cinnamon in a large bowl. Mix until incorporated.
In a separate bowl, beat brown sugar and butter until fluffy. Add eggs in, one at a time, until beaten. Whisk in vanilla extract until smooth. Add flour mixture to sugar mixture alternately with Greek yoghurt, beginning and ending with flour mixture.
If the batter is thick, stir in 2 Tbsp. milk. Fold in chopped dates. Spoon batter into muffin tin liners, filling ¾ of the way to the top. In a small bowl, combine streusel ingredients.
Top each muffin with 1 Tbsp. streusel. Place in oven and bake 22-27 minutes, until toothpick in centre comes out clean. Cool muffins for 10 minutes and remove from pan.
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