Preheat oven to 350 degrees.
Line a 9"x5" loaf pan with parchment paper and lightly grease.
In a large mixing bowl, cream together the butter and brown sugar until light and fluffy, about 2–3 minutes.
Add maple syrup, eggs, and vanilla extract. Mix until smooth.
Add flour, baking powder, baking soda, and cinnamon. Alternate adding the sour cream and milk, mixing gently until combined.
Fold in the chopped walnuts making sure not to overmix.
Spread the batter evenly into the prepared loaf pan.
In a small bowl, make the streusel topping by combining flour, walnuts, brown sugar, and cinnamon. Add softened butter and mix with a fork until crumbly.
Sprinkle the streusel evenly over the top of the cake batter.
Bake for 55–65 minutes, or until a toothpick inserted into the center comes out clean. (If the loaf browns too quickly, loosely tent with foil during the last 15 minutes of baking.)
Cool in the pan for 15 minutes before transferring to a wire rack.
If you want to add the glaze, whisk together powdered sugar, maple syrup, and milk until smooth. Drizzle over the cooled loaf.