Preheat the oven to 350 F. Grease and flour a 9” square or round cake pan.
Place the butter in a large glass or heatproof bowl and microwave it until it’s almost but not completely melted. You will still have some lumps. Or place is a small saucepan and melt over low heat. Remove from heat before it is completely melted.
Add the sugar and whisk very well, a full 30 seconds.
Add the salt and vanilla and whisk for another 30 seconds.
Add the eggs one by one, whisking to fully incorporate before adding another then whisk for a full minute, until it’s light and creamy.
Sift in the cake flour, powdered sugar and baking powder together and add to creamed mixture.
Whisk in the sour cream. Fold the batter until it’s smooth.
Pour the batter into the prepared pan. Sprinkle with an additional 3 tablespoons of granulated sugar.
Bake for 30-45 minutes, until a cake tester comes out clean and the top is golden brown.