You can make these up to a day ahead and keep in an airtight container. They are buttery delicious and very moist!
- Set oven to 350 degrees.
- Set oven rack to middle position.
- Generously butter the bottoms, sides and tops of 18 non-stick muffin tins.
- In a large bowl stir together sugar, flour, salt and 3/4 cup brown sugar.
- Add in 3/4 cups cold butter and blend with a pasty blender or you fingers until the mixture resembles coarse meal with some pea-size butter pieces in the mixture; transfer 3/4 cup of the mixture to a bowl and blend in cinnamon, the remaining 1/4 cup butter and remainder 1/4 cup brown sugar with your fingertips or pastry blender until crumbly.
- Stir in the chopped nuts; chill for 15 minutes.
- Whisk together sour cream, egg, egg yolk, vanilla, baking soda.
- Add in the flour mixture until JUST combined (the batter will be stiff).
- Divide the batter evenly between the 18 muffin tins (they should be about two-thirds full).
- Sprinkle each with streusel topping (pressing it lightly into the batter).
- Bake coffeecakes for about 20-25 minutes, or until they test done with a toothpick inserted in the middle.
- Cool in the pans on racks for 30 minutes.
- Loosen the cakes with a small sharp knife, then carefully remove cakes from the muffin tins.