Preheat oven to 350-degrees. Line the bottom and sides of a 9x13-inch pan with aluminum foil or parchment paper, making sure to overlap the sides to create handles. Cover with nonstick spray.
Sift together flour, cocoa, and salt (and espresso powder, if using) in a small bowl and set aside.
Melt chocolate and butter together in a double boiler or in slow increments in microwave. Stir until smooth. Whisk in sugars and stir until dissolved and mixture has cooled slightly, then add eggs and vanilla extract until just combined.
Fold together the chocolate and flour mixes until just incorporated. Pour into prepared pan.
Bake for 25 to 30 minutes or until toothpick inserted into the middle comes out clean. Transfer pan to a wire rack and allow to cool completely.
Cream filling stage
Beat together corn syrup, butter, vanilla, and salt on medium-high speed until smooth. Add powdered sugar, a little bit at a time, mixing until creamy.
Dump 3/4 of the cream mixture on top of cooled brownies and spread into an even layer. Add a drop of yellow food coloring to remaining cream mixture and stir until evenly colored. Drop dollops of yellow cream on top of white layer, and then swirl gently with a spatula. Refrigerate for at least 2 hours or until set.
Slowly melt the chocolate and butter together in a double boiler or microwave bowl. Stir until smooth. Pour over cream filling, carefully spreading into a thin, even layer.
Chill in the fridge long enough for the chocolate to not quite harden, 15-20 minutes, OR let it chill for several hours and then let set at room temperature for about 15 minutes. The chocolate needs to be soft enough to cut through without cracking, but not soft enough to smear with each cut.
Use the foil/paper to lift the entire block out of the pan and onto a cutting board. Using a large sharp knife, cut into 2-inch squares. Keep in sealed container in fridge; brownies will keep upward of a week.
OM NOM NOM!