Position an oven rack to the center of your oven and preheat the oven to 35oF [180C]. 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.
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.
Add the flour, baking powder, and baking soda. Whisk until well-combined and smooth.
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 to the rack to cool almost completely.