Turn on the oven to 425°F. Pour the butter into a small baking pan and pour into the oven.
In a large mixing bowl combine the flour with the sugar and the salt. In another bowl beat the egg with the egg white and the milk. Stir in the flour mixture. Whisk well to combine. Divide the mixture in two parts. Add the cocoa powder into one part and whisk.
Remove the baking pan from the oven. The butter will be already dissolved. Pour the cocoa mixture into the hot baking pan. Pour the white mixture in the center of the cocoa mixture and swirl with a wooden stick or a knife.
Pour into the oven and bake about 10-15 minutes. The pancake will rise during the baking time but will puff when you remove it from the oven.
Dust with confectioners' sugar if you like and serve warm with melted chocolate or honey.