Make the topping:
Position a rack in the middle of the oven and heat the oven to 350°F. In a large saucepan, melt all the butter; remove from the heat. Pour 1/2 cup into a measuring cup and reserve for mixing the cake batter. Add the sugar, brown sugar, cinnamon, nutmeg, and salt to the remaining 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 pan. In a large bowl, whisk together the flour, sugar, baking powder, and salt. In a medium bowl, whisk the egg, milk, reserved 1/2 cup melted butter, and vanilla 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. Cool on a rack and dust with the confectioners’ sugar. Serve warm or at room temperature.
Photo: Amy Albert