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 3/4 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 confectioners’ 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.