Preheat the oven to 350F. butter a 9x5 loaf mold or round mold and coat lightly with flour, or use a no-stick bakery spray.
Soften the butter in a bowl until it becomes creamier. Add the sugar 2 to 3 times and whisk vigorously until a nice cream is created.
Separate the egg yolk from the white. Add the egg yolks to the butter-sugar mixture. Incorporate the sifted flour.
In another bowl beat at full speed the egg whites with a pinch of salt. Once soft peaks are formed and no more white liquid is in your bowl from the egg white, Incorporate the egg whites into the egg flour mixture.
Pour the mixture into the mold and bake for 40 minutes in the middle of the oven.
Verify that it is ready by inserting a knife in the middle.
Remove from the oven and wait 5 minutes prior to removing from the mold.