For the cake:
12 tablespoons (1 1/2 sticks) unsalted butter at room temperature
1 1/2 cups granulated sugar
3 extra-large eggs at room temperature
1 1/2 teaspoons pure vanilla extract
1 1/4 cups sour cream
2 1/2 cups cake flour (not self-rising)
2 teaspoons baking powder
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
For the streusel:
1/4 cup light brown sugar, packed
1/2 cup all-purpose flour
1 1/2 teaspoons ground cinnamon
1/4 teaspoon kosher salt
3 tablespoons cold unsalted butter, cut into pieces
3/4 cup chopped walnuts, optional
For the glaze:
1/2 cup confectioners' sugar
2 tablespoons real maple syrup
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.