Step 1

Position a rack in the center of your oven and preheat the oven to 350ºF. Butter or coat an 8-inch square baking pan with nonstick spray. Line the pan with a strip of parchment paper that hangs over two of the edges.

Step 2

Make the cake: In a large bowl, whisk the granulated sugar and eggs until pale and foamy, about 1 minute. Add the sour cream, butter, nutmeg, vanilla, and salt. Whisk until smooth and emulsified.

Step 3

Add the flour, baking powder, and baking soda. Whisk until well-combined and smooth.

Step 4

Pour the batter into the pan and bake the cake until puffed and golden, and a skewer inserted into the center comes out clean, 25 to 35 minutes. Set the pan on a rack to cool for about 15 minutes. Then use the parchment paper to lift the cake out of the pan and set it on the rack to cool almost completely.

Step 5

Finish the cake: While the cake is just warm to the touch, brush the top with the melted butter and dust with the confectioners’ sugar. You should have a nice thick layer of confectioners' sugar—more than you think might be necessary. (Store the cake, covered, at room temperature for up to three days. The cake will absorb the sugar on top, so it might need a fresh dusting of confectioners’ sugar after the second day.)