Preheat oven to 350 F. If not using a non-stick bundt/tube pan, grease and flour the pan.
In medium bowl, sift together the cake flour, baking powder, baking soda, and salt.
In large bowl, cream the butter and sugar with an electric mixer until light and creamy, about 4-5 minutes.
Add eggs, one at a time, followed by the vanilla and sour cream.
With mixer on low, add the flour mixture to the batter until just combined.
To make the streusel, place all ingredients except the walnuts in a small bowl and pinch with your fingers until it forms a crumble.
Mix in the walnuts.
To assemble the cake, spoon half of the batter into the pan and even out with a spatula or knife.
Sprinkle 3/4 cup streusel, then spoon the rest of the batter into the pan.
Spread it evenly and top with the remaining streusel. If you make this in a bundt pan, this streusel layer will be on the bottom, but if you make it in a tube pan, it will be on the top.
It didn't really matter to me, but if you want the streusel-topped look, make it in a tube pan.
Bake for 50-60 minutes. I took mine out at 50 and it was perfectly done.
Let the cake cool for at least 30 minutes, then carefully flip the cake out onto a serving plate.
Whisk together the confectioners' sugar and maple syrup and add desired amount of milk to make the glaze thinner.
Drizzle glaze over the cake with a spoon. Serve and enjoy!