Make the cake: Preheat the oven to 350 degrees F. Coat a 9-inch springform pan with cooking spray. Whisk the flour, baking powder, nutmeg, cinnamon, baking soda and salt in a medium bowl. Beat the butter and granulated sugar in a large bowl with a mixer on medium-high speed until light and fluffy, about 3 minutes. Beat in the eggs, egg yolk and vanilla until combined.
Reduce the mixer speed to low and add the flour mixture in three batches,
alternating with the sour cream in two batches. Increase the mixer speed to medium high and beat until well combined, about 30 seconds. Transfer the batter to the prepared pan; set aside.
Make the topping: Combine the flour, brown sugar, cinnamon, nutmeg and salt in a medium bowl. Work in the butter with your fingers until clumpy. Scatter the topping over the cake batter. Bake until golden brown and a toothpick inserted into the center comes out clean, 1 hour to 1 hour 10 minutes. Transfer to a rack and let cool 20 minutes, then remove the springform ring and let cool completely.
Make the glaze: Whisk the confectioners' sugar with 3 tablespoons eggnog in a medium bowl until smooth. If the glaze is too thick, gradually whisk in the remaining 1 tablespoon eggnog as needed. Drizzle the glaze over the cake before serving.
Photograph by Levi Brown