Preheat the oven to 350°F (175°C) and grease a 9-inch square baking pan or line it with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a large bowl, beat the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.
Add the dry ingredients alternately with the apple cider and sour cream, beginning and ending with the dry ingredients.
Mix gently until just combined; do not overmix the batter.
Spread the batter evenly into the prepared baking pan.
To make the crumb topping, combine the flour, brown sugar, cinnamon, and salt in a bowl.
Pour in the melted butter and stir until large, moist crumbs form.
Sprinkle the crumb mixture evenly over the cake batter.
Bake for 40–45 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for at least 15 minutes before slicing.
If desired, whisk together the powdered sugar, apple cider, and vanilla and drizzle the glaze over the cooled cake.
Slice and serve with coffee, tea, or a glass of cold milk.
