Preheat the oven to 375 °F (190 °C). Line two 6-cup 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.
Notes
Make sure the bananas are very ripe (black even).
If you are using frozen bananas for this recipe, please make sure do the following:
defrost 4 bananas (instead of 3)
peel the bananas and place in a strainer set over a bowl
press very gently to squeeze out the liquid
place the liquid in a saucepan and boil it down to thicken it, stirring often so that it doesn't burn. Add this banana concentrate to the bananas and proceed with recipe.
If you want to make a variation on these banana muffins, add any of the following:
add 250 mL (1 cup) chocolate chips instead of the chopped dates