To make the coffee cake

1. Preheat the oven to 350°F. Using a hand-held or standing mixer, beat the butter with the sugar until light and fluffy. Beat in the sour cream, followed by the eggs, one at a time. Add the vanilla.
2. Combine the flour, baking powder, baking soda, and salt. Add the flour mixture to the butter in two additions and mix until just incorporated, taking care not to over-mix.
3. Grease a 10” skillet or baking pan very well with baking spray or butter. Spread half of the coffee cake batter on the bottom of the pan. Sprinkle half the streusel evenly on top. Spread the other half of the batter on top of the streusel and evenly spread out. Top with the remaining streusel.
4. Bake until a tester inserted into the center comes out clean and the streusel is golden brown, about 40-45 minutes.

To make the streusel

1. Mix the flour, brown sugar, and cinnamon in a bowl. Using a pastry cutter or large fork, cut in the cold butter pieces until it resembles small, pea-sized pieces. Stir in the walnuts and a pinch of salt and combine. The streusel should clump together when you press it.