By An Edible Mosaic
Prep Time: 10 min
Cook Time: 35 min
In a large bowl, whisk together the cocoa powder, espresso, and boiling water. Add the chopped chocolate and whisk until melted. Whisk in the butter and oil, and then cool until lukewarm.
Whisk in the egg, egg yolk, and vanilla until smooth.
Whisk in the sugar. Stir or whisk in the flour, salt, and baking soda until just combined.
If using, stir in the nuts.
Pour the batter into a prepared baking dish. Bake until a toothpick inserted inside comes out moist with a couple crumbs, about 30 to 40 minutes. Be careful not to over-bake! Cool completely before cutting.