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 alternatively with Greek yogurt, beginning and ending with flour mixture. If batter is thick, stir in 2 Tbsp. milk. Fold in chopped dates. Spoon batter in to 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 center comes out clean. Cool muffins for 10 minutes and remove from pan.