Preheat oven to 350ºF and place the rack in the middle.
MAKE THE TOPPING:
In a large saucepan, melt all of the butter; remove from heat. Pour 1/2 cup into a measuring cup and reserve for mixing into the cake batter.
Add the sugar, brown sugar, cinnamon, nutmeg and salt to the remaining melted butter in the saucepan.
Stir with a fork, pressing when needed, until there are no lumps of sugar.
Add the flour and stir gently until well blended and crumbly. Set aside.
MAKE THE CAKE:
Lightly butter a 9x13 inch baking dish.
In a large bowl, whisk the flour, sugar, baking powder, and salt.
In a medium bowl, whisk the egg, milk, vanilla, and reserved 1/2 cup melted butter until combined.
Pour the liquids over the dry ingredients and gently stir until just blended.
Scrape the batter into the prepared pan and spread evenly.
Break up the crumb mixture with your fingers and sprinkle it evenly and generously over the cake batter.
Bake until the top is lightly browned, the cake springs back when lightly pressed in the center, and a pick inserted in the center comes out clean, about 35 minutes.
Let cool on a rack and dust with confectioners' sugar.
Serve warm or at room temperature.