Make the streusel by mixing the sugar, brown sugar, walnuts, vanilla, and cinnamon together in a small bowl. Set it aside.
Preheat the oven to 350 degrees F. Generously brush a round 9 by 2-inch deep cake pan with butter. In a medium bowl whisk the flour, baking soda, salt, nutmeg and mace together and set it aside.
In a standing mixer fitted with the paddle attachment, or with an electric hand-held mixer, cream the butter and sugar on medium speed until light and fluffy. Add the eggs, one at a time, mixing until incorporated. Mix the vanilla with the sour cream. Add the flour mixture in 3 parts to the butter mixture, alternating with the sour cream in 2 parts until just combined.
Spread half the batter into the prepared pan and sprinkle with half the streusel. Spoon the remaining batter on top and spread it out with a spatula. Scatter the remaining streusel over the top of the batter. Bake the coffee cake for 1 hour and about 10 minutes, or until golden brown and a toothpick inserted into the center of the cake comes out clean. Cool on a rack for 10 to 15 minutes before serving.
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