You can't go wrong with a classic -- and this one hits all the right notes. Warm spices like cinnamon, nutmeg and mace are nicely offset by the richness and tang of sour cream. Walnuts add crunch to the surprise layer of streusel inside the cake.
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.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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