Preheat oven to 350ºF. Line a rimmed 9 x 13-inch baking sheet with parchment paper.
Melt chocolate with the butter.
In the bowl of an electric mixer, whip eggs and sugar at high speed for 5-6 minutes.
Carefully pour in melted chocolate mixture. Sift in flour, cocoa, and salt. Mix until mixture is smooth and shiny, about 4 minutes. Add in extracts, if using. Fold in white chocolate.
Pour batter into prepared baking sheet. Bake for 25 minutes (the edges should be hard, top cracked, and middle soft).
Prepare the frosting: Melt chocolate over a double boiler. Add praline paste and mix until combined. Gently pour over the top of the cake. Tilt the pan sideways until entire cake is fully covered. Sprinkle nuts or chocolate curls on top. Refrigerate for 15 minutes, until frosting is set. Let cool before slicing into bars.
Recipe by Lil' Miss Cakes at https://lilmisscakes.com/2015/secret-restaurant-recipes-cookbook-review-and-giveaway