01 -
Preheat oven to 350°F (175°C) and lightly grease a 7x5-inch baking dish.
02 -
In a small bowl, whisk together brown and granulated sugars, cinnamon, nutmeg, and salt. Drizzle in melted butter and mix until well combined. Gradually incorporate flour until completely integrated. Refrigerate until needed.
03 -
In a small bowl, whisk together flour, baking powder, and salt. Set aside.
04 -
In a medium bowl, cream butter and granulated sugar until light and fluffy, about 1 minute. Beat in egg and vanilla until well-combined. Incorporate sour cream. Fold in half the flour mixture, followed by 1 tablespoon of milk. Repeat with remaining flour mixture and milk until just combined.
05 -
Pour batter into prepared dish and crumble streusel mixture over the top in pea-sized pieces. Bake for 28-34 minutes, until a toothpick inserted into the center comes out with just a few crumbs.
06 -
Allow cake to cool for at least 15 minutes before cutting and serving.