Heat oven to 350°F. Grease and line a 9x13-inch pan with parchment.
Stir brown sugar, granulated sugar, flour, cinnamon, nutmeg, and salt for the crumbs.
Drizzle in melted butter and toss until clumps form. Chill the crumbs while you make batter.
Whisk flour, baking powder, baking soda, salt, and nutmeg for the batter.
Cream softened butter and sugar until light and fluffy.
Beat in the eggs, vanilla, and rum extract until smooth.
Mix in half the dry mix, then eggnog and sour cream. Finish with remaining dry mix.
Spread batter in the pan. Scatter crumbs evenly on top and press lightly to adhere.
Bake 35–42 minutes, until a tester comes out clean and crumbs are golden.
Cool on a rack for 20 minutes.
Whisk powdered sugar, eggnog, vanilla, and nutmeg into a thick glaze. Drizzle over warm cake.
Notes
Try stirring 1/2 cup chopped pecans into the crumb topping for crunch, or add a splash of bourbon to the glaze. Store covered at room temperature for 2 days, or refrigerate up to 5 days. The cake freezes well for 2 months without the glaze.This recipe is an original creation inspired by classic Easy Eggnog Crumb Cake Recipe flavors. All ingredient ratios and instructions are independently developed.