Make the streusel topping first by combining the brown sugar, pecans and cinnamon in a small bowl. Set aside.
In a big bowl, beat the butter and white sugar until light and fluffy; add the eggs, one at a time, beating well after each.
In a separate bowl, whisk together the dry ingredients: flour, baking powder, baking soda, nutmeg and salt. Slowly add the flour mix to the eggs and butter. Pause to add in some sour cream, then more flour, going back and forth until it's all blended.
Apply nonstick spray to a 9x13-inch pan. Pour in the batter and spread to the edges. Sprinkle the streusel all over the top.
Cover well with plastic wrap and chill overnight, or up to 18 hours.
Preheat the oven at 350-degrees. Remove the plastic wrap. Bake the cake for 35 minutes, or until a toothpick comes out clean. Serve warm or at room temperature.
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