Preheat the oven to 400 degrees F. Prepare the muffin pan by either buttering the cups or using paper liners. This recipe makes 18 muffins, so you will need to use several muffin pans.
Place the walnuts on a baking sheet lined with foil or parchment paper and toast the walnuts in the oven for about 5-8 minutes, stirring a couple of times until they are a bit darker and smell toasty.
If you are using whole or half walnuts, place the nuts on a cutting board and chop them into smaller pieces with a chef's knife.
In a large mixing bowl melt the butter in the microwave. Measuring the milk then add the eggs to the measuring cup and beat with a fork. Using a wire whisk mix in the maple syrup and the milk and eggs with the melted butter.
Add the flour, baking powder, and salt into the wet ingredients and whisk or stir until the flour is combined.
Add the chopped walnuts to the mixing bowl and stir until combined.
Let the batter rest for 30 minutes. then spoon or scoop into the muffin pans making them 3/4 full. Note: this is optional, as you can just go ahead and scoop the batter into the pan.

Combine all the softened butter, flour, and brown sugar into a medium-sized mixing bowl. Using a pastry cutter mix the ingredients until coarse and crumbly. You can also use two knives and cut the butter in a criss-cross fashion to make the crumbles.
If your kitchen is hot, it is helpful to chill the streusel for 5-10 minutes to firm the butter up and this makes it easier to sprinkle on top of the batter.
With your fingers, sprinkle the streusel topping over the batter of each muffin.
Bake at 400 degrees F. for 10 minutes then reduce the oven temperature to 375 degrees F and bake for 8-10 minutes. The muffins should be golden brown and the center should be dry when tested with a toothpick.
Remove the muffin pans from the oven and let cool for 5 minutes then remove each muffin to cool on a cooling rack.
If you did not use paper muffin liners let them cool for five minutes then run a knife around the edges of the muffins to loosen the muffins from the cups and set them on a wire rack to cool.
Just don't let the muffins cool in the pan as this can make the muffin buttom hard and crusty.

Combine all the glaze ingredients in a small mixing bowl then drizzle over the cooled muffins.

After the muffins have cooled completely and the glaze has hardened, place the muffins in an airtight container. These muffins will keep for 3-4 days.
You can freeze these muffins for several months. When ready to eat let the muffins thaw on the counter at room temperature..