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1
Make the topping:.
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2
Position a rack in the middle of the oven and heat the oven to 350F In a large saucepan, melt all the butter; remove from the heat.
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3
Pour 1/2 cup into a measuring cup and reserve for mixing the cake batter.
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4
Add the sugar, brown sugar, cinnamon, nutmeg, and salt to the remaining butter in the saucepan.
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5
Stir with a fork, pressing when needed, until there are no lumps of sugar.
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6
Add the flour and stir gently until well blended and crumbly.
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7
Set aside.
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8
Make the cake:.
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9
Lightly butter a 9x13-inch baking pan.
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10
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
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11
In a medium bowl, whisk the egg, milk, reserved 1/2 cup melted butter, and vanilla until combined.
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12
Pour the liquids over the dry ingredients and gently stir until just blended.
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13
Scrape the batter into the prepared pan and spread evenly.
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14
Break up the crumb mixture with your fingers and sprinkle it evenly and generously over the cake batter.
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15
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.
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16
Cool on a rack and dust with the confectioners sugar.
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17
Serve warm or at room temperature.
