Prepare a 1/2 sheet tray with parchment and pan spray.
Melt the butter and chocolate together over a double boiler.
Separate the eggs and whisk the egg yolk and sugars together into a paste, add the butter chocolate mixture and stir to combine.
Add flour and salt and fold in with a spatula.
Whip the whites to medium peaks, then fold into the batter.
Spread into the prepared half sheet.
Bake at 350 for 10-15 minutes until set.
Allow to cool in the pan.