Heat the oven to 375°F (190°C). Line a 12-cup muffin pan with paper liners or grease the cups lightly so the baked muffins release easily.

Make the optional streusel by mixing the brown sugar, flour, and cinnamon in a small bowl. Rub in the cold butter with your fingertips until coarse crumbs form, then refrigerate the topping while preparing the batter.

Whisk the flour, baking powder, cinnamon, and fine salt together in a medium bowl. Mixing the dry ingredients first distributes the leavening and spice evenly throughout the muffins.

In a separate bowl, whisk the melted and slightly cooled butter, maple syrup, brown sugar, eggs, milk, and vanilla until smooth. The butter should not be hot, since very hot butter can affect the eggs.

Add the dry ingredients to the wet ingredients and fold just until no dry flour remains. A few small lumps are acceptable; excessive mixing can develop gluten and make the muffins dense.

Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full. Sprinkle the chilled streusel over the tops if using.

Bake for 18–22 minutes, beginning to check at 18 minutes. The tops should spring back lightly, and a toothpick inserted in the center should come out clean or with a few moist crumbs.

Cool the muffins in the pan for 5 minutes, then transfer them to a wire rack. Allow them to cool further before glazing so the icing stays on top instead of melting away.

For the optional glaze, whisk the confectioners’ sugar and maple syrup together. Add milk one teaspoon at a time until the mixture is smooth and pourable, then drizzle it over the cooled muffins.