Add eggnog, sour cream, eggs, and rum extract. Mix until combined.
Add flour, baking powder, baking soda, salt, and nutmeg. Mix just until no dry streaks remain.
Spread batter evenly in the prepared pan.
Mix streusel ingredients until crumbly.
Sprinkle streusel over the batter.
Bake until a toothpick inserted in the center comes out clean.
Cool cake before icing.
Whisk powdered sugar and eggnog into a smooth drizzle.
Drizzle icing over cooled cake and let set before serving.
Notes
Measure flour carefully so the cake does not turn dense. Room-temperature ingredients mix more evenly. Rum extract helps the cake taste like eggnog instead of plain vanilla nutmeg cake. Store covered at room temperature up to 3 days or freeze individual pieces up to 2 months.