Position the rack in the center of the oven. Preheat 350. Lightly butter a 9x13 baking pan, set aside.
Make the topping: In a large saucepan melt butter for both the crumb topping and the cake and set aside 1/2 cup for the cake batter. Add sugar, brown sugar, cinnamon, nutmeg and salt to the remaining butter in the saucepan. Stir with a fork, until there are no lumps. Add the flour and stir gently until well blended and crumbly. Set aside.
For the cake: In a large bowl, whisk the flour, sugar, baking powder and salt. In a medium bowl, whisk egg, milk, vanilla and reserved butter until combined. Pour the liquid over the dry i9ngredients and gently stir until just blended. Scrape the batter into the prepared pan and spread evenly.
Break up the crumb mixture with fingers and sprinkle it evenly and generously over the cake batter. Bake until the top is lightly browned, and the cake springs back when lightly pressed in the center, and a pick inserted in the center comes out clean; about 35 minutes.
Let the cake cool on a wire rack and dust with the powdered sugar. Serve warm or at room temperature.