- Preheat oven to 300 degrees. Line a 9 x 13 pan with an oversized piece of parchment paper.
- Combine sugars and butter in a saucepan over medium heat. Using a whisk, stir until combined. Sugar will still be a bit grainy, but mixture should be somewhat smooth.
- Remove from heat and allow mixture to cool slightly. Add chopped chocolate and whisk until smooth. Note: if the butter mixture is too warm chocolate could break and separate, so be careful.
- Next add eggs one at a time, whisking until thick and shiny. Add vanilla, salt, cayenne, and espresso. Stir until smooth.
- Add flour and whisk slowly until smooth. Pour batter into pan lined with parchment paper.
- Bake on center rack for about 35 minutes. The center will puff a bit and the edges will look shiny and set somewhat firm.
- Remove pan from oven and allow to cool at room temperature for 45 minutes. Place in the refrigerator for 1 hour before cutting.