Combine all ingredients in a food processor or blender.
Set processor on whip; whip until blended and powdered.
Stir and reprocess until completely blended.
Store any leftover in an air tight container
Instructions
Preheat the oven to 350.
Grease and flour a 10-inch tube pan.
Cream the butter and sugar in the bowl of stand mixer fitted with the paddle attachment until light in color (about 4-5 minutes).
Add the eggs 1 at a time mixing well after each addition.
Add in the vanilla and sour cream mixing just until incorporated.
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. You do not want to incorporate air into this batter.
Next make the streusel: Add the brown sugar, flour, cinnamon, salt, and butter in a bowl and using a pastry cutter, cut it until it forms a crumble.
At this point you can add in the nuts.
Spoon half the batter into the pan and spread it out with rubber spatula.
Next sprinkle half the streusel on top of the batter.
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 (middle rack), until a wooden skewer comes out clean.
Allow the cake to cool on a wire rack for at least 30 minutes.
Place your serving plate on top of the pan and carefully flip it over while pressing the plate and pan together.
Lift up the pan and the cake will be streusel side up.
Make the glaze: Whisk together the confectioners’ sugar, brown sugar, cinnamon and maple syrup together, adding a few drops of milk if necessary, to make the glaze thinner.