Make the streusel in a food processor, minus the nuts, which are stirred in after pulsing.
Cream the butter and sugar, then add the eggs, vanilla, and sour cream.
Combine the flour, baking powder, baking soda, and salt.
Slowly mix the dry ingredients with the wet ones until combined.
Pour half the batter into the baking pan, sprinkle in half the streusel, then add the rest of the batter followed by the last of the streusel.
Bake the cake at 350°F for 50 minutes.
Once cooled, plate it crumble-side-up, and drizzle it with freshly-whisked maple glaze.
