Preheat the oven to 375 °F. Line muffin pans with paper liners to make 12 muffins. Set aside.
In a large bowl, whisk together the flour, brown sugar, oats, baking powder, salt, and cinnamon.
Add the butter, and work it into the dry ingredients to form a crumbly mixture that looks a little like coarse, wet sand.
Scoop out 60 mL (¼ cup) of this mixture and set it aside as a crumble topping for the muffins.
In a separate bowl, whisk together the eggs, mashed banana, and vanilla.
Mix the wet ingredients with the dry ingredients, scraping the bottom and sides to incorporate everything.
Fold in the chopped pitted dates.
Scoop the batter into the prepared muffin pans. Scatter a little of the reserved crumble over each muffin and sprinkle with a few oats.
Bake for 25–30 minutes, until the edges are a golden brown and a cake tester inserted into the middle comes out clean.