Preheat the oven to 350 degrees F. Grease and flour a 10-inch tube pan.
Cream the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment for 4 to 5 minutes, until light. Add the eggs 1 at a time, then add the vanilla and sour cream. In a separate bowl, sift together the flour, baking powder, baking soda, and salt. With the mixer on low, add the flour mixture to the batter until just combined. Finish stirring with a spatula to be sure the batter is completely mixed.
For the streusel, place the brown sugar, flour, cinnamon, salt, and butter in a bowl and pinch together with your fingers until it forms a crumble. Mix in the walnuts, if desired.
Spoon half the batter into the pan and spread it out with a knife. Sprinkle with 1 1/2 cup streusel. Spoon the rest of the batter in the pan, spread it out, and scatter the remaining streusel on top. Bake for 50 to 60 minutes, until a cake tester comes out clean.
Let cool on a wire rack for at least 30 minutes. Carefully transfer the cake, streusel side up, onto a serving plate. Whisk the powdered sugar and maple syrup together, adding a few drops of water if necessary, to make the glaze runny. Drizzle as much as you like over the cake with a fork or spoon.