03Cream 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 one at a time, then add the vanilla and sour cream.
04In a separate bowl, sift together the flour, baking powder, baking soda, and salt.
05With the mixer on low, add the flour mixture to the batter until just combined.
06Finish stirring with a spatula to be sure the batter is completely mixed.
07For 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.
08Mix in the walnuts, if desired.
09Spoon half the batter into the pan and spread it out with a knife.
10Sprinkle with 3/4 cup streusel.
11Spoon the rest of the batter in the pan, spread it out, and scatter the remaining streusel on top.
12Bake for 50 to 60 minutes, until a cake tester comes out clean.
13Let cool on a wire rack for at least 30 minutes.
14Carefully transfer the cake, streusel-side up, onto a serving plate.
15Whisk 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.
16**Cook time does not include cooling the cake for 30 minutes on wire rack.