Toast walnuts:

Heat oven to 400°F (205°C). Put parchment on a baking sheet. Arrange walnuts in one layer and bake 7-8 minutes until they smell fragrant and turn golden. Let cool.

Sift dry ingredients:

In a big bowl, sift together flour, baking powder, cinnamon, nutmeg, and salt.

Mix wet ingredients:

In a medium bowl, combine brown sugar with melted butter using a mixer. Add eggs, maple syrup, milk, maple extract, and vanilla; beat until smooth.

Combine batter:

Alternately add dry mix and milk to the wet ingredients in two batches, stirring only until combined. Gently stir in 1 cup (112g) of the toasted walnuts.

Fill muffin cups:

Put liners in a 12-cup muffin pan. Split the batter evenly among the cups.

Make streusel topping:

In a small bowl, stir together flour, brown sugar, granulated sugar, melted butter, toasted walnuts, cinnamon, and hazelnut extract until the mixture forms clumps. Scatter evenly over the muffin batter.

Bake muffins:

Bake at 400°F (205°C) for 5 minutes, then lower the temperature to 375°F (190°C) without opening the oven. Continue baking for 16-17 minutes or until a toothpick inserted in the center comes out clean.

Glaze muffins:

While muffins bake, whisk icing sugar, milk, maple syrup, and maple extract together until the glaze is smooth. Cool muffins fully, then drizzle the glaze over them. Wait a few minutes for it to set.