1 cup semisweet chocolate chips
1 1/2 sticks (170 grams) unsalted butter, cut into pieces
4 ounces (113 grams) unsweetened chocolate, chopped
1 1/2 cups granulated sugar
3 large eggs plus 1 egg yolk
1 teaspoon vanilla extract
3/4 cup all-purpose flour
1/4 teaspoon fine sea salt
Preheat oven at 325°F. Prepare your baking pan with oil.
Melt the chocolate chips, unsweetened chocolate with butter over a double boiler or microwave until smooth and no chunks left. Set aside and let it cool.
While the chocolate is melting, whip the egg whites and the sugar in a large mixer fitted with a paddle for about 10 minutes until thick.
Add the melted chocolate and vanilla extract and the egg-sugar mixture. Stir to blend completely. (You can do this manually but preferred to have a hand mixer).
You can then add the flour. Spread the butter evenly onto a parchment lined and buttered sheet pan.
Bake at approximately 160C (325F) for 40 minutes, rotating the pan after the first 20 minutes. Make sure not to over bake it.
Allow it to cool and slice. Serve and enjoy.