1- First, preheat your oven to 350°F (175°C) and thoroughly grease a 9×9 inch baking pan. This ensures your cake won’t stick and will release easily after baking.

2- In a medium bowl, cream together the softened butter, packed brown sugar, granulated sugar, egg, and vanilla until the mixture is smooth and well combined. This step incorporates air into the batter, which helps create a lighter texture in the finished cake.

3- Next, add the all-purpose flour, salt, and baking powder to the bowl. Mix until just combined, being careful not to overmix at this stage. The mixture may appear crumbly, which is normal at this point.

4- Once the dry ingredients are incorporated, stir in the eggnog and sour cream all at once. Mix until the batter becomes smooth and creamy. The addition of these wet ingredients will transform the crumbly mixture into a cohesive cake batter.

5- Pour the batter into the prepared baking pan, spreading it evenly with a spatula. Set aside while you prepare the crumb topping.

6- In a separate medium bowl, combine the softened butter, packed brown sugar, all-purpose flour, cinnamon, and nutmeg for the topping. Using your fingers or a pastry cutter, cut the butter into the dry ingredients until coarse crumbs form. Be careful not to overmix the topping, as you want to maintain a crumbly texture rather than turning it into a paste.

7- Sprinkle the crumb topping evenly over the batter in the baking pan, gently pressing it down slightly to adhere it to the batter.

8- Place the pan in the preheated oven and bake for 40 to 45 minutes, or until a toothpick inserted into the center of the cake comes out clean. The top should be golden brown, and the edges may start to pull away slightly from the sides of the pan.

9- While the cake is baking, you can prepare the glaze. In a small bowl, whisk together the powdered sugar, eggnog, cinnamon, and nutmeg. The consistency should be pourable but not too thin. If needed, adjust by adding more powdered sugar to thicken or more eggnog to thin.

10- Once the cake is done, remove it from the oven and let it cool in the pan for at least 30 minutes. This cooling period is important as it allows the cake to set and makes it easier to handle.

11- After the cake has cooled, drizzle the glaze over the top. You can use a spoon to create an artistic pattern or simply pour it evenly across the surface.

12- Slice and serve your eggnog coffee cake while still slightly warm for the best flavor and texture. The combination of the moist cake, crunchy topping, and creamy glaze creates a delightful experience with every bite.