Preheat your oven to 350°F (175°C) to ensure even baking.
Grease your 7x5-inch baking dish with butter or non-stick spray.
In a small bowl, combine the brown sugar, granulated sugar, cinnamon, nutmeg (if using), and 1/2 teaspoon of salt. Mix well and set aside.
In a medium mixing bowl, whisk together the 1/2 cup + 1 tablespoon all-purpose flour, 1/2 cup + 2 tablespoons all-purpose flour, baking powder, and the remaining 1/4 teaspoon of salt. Set this bowl aside.
In a large mixing bowl, beat the softened butter and 1/4 cup granulated sugar together until light and fluffy, about 2-3 minutes.
Add the egg and vanilla extract to the butter-sugar mixture. Beat until fully incorporated.
Mix in the sour cream and milk until the batter is smooth.
Gradually add the dry ingredient mixture to the wet ingredients. Mix until just combined; be careful not to overmix.
Pour half of the batter into the prepared baking dish, smoothing it out. Sprinkle half of the topping mixture over the batter. Pour the remaining batter on top and finish with the rest of the topping.
Bake for about 25-30 minutes or until a toothpick inserted into the center comes out clean.
Once baked, let the Mini Coffee Cake cool in the dish for about 10 minutes. Then, transfer it to a wire rack to cool completely. Slice and serve.