Heat the oven to 350° F. Line an 8 x 8 glass baking dish with aluminum foil. Spray lightly with cooking spray.
In a small saucepan, combine the olive oil and chocolate; cook over medium-low heat, whisk vigorously, until combined and smooth. Remove from heat and bring to room temperature.
In a large bowl, combine the eggs, sugar, cocoa, vanilla, and salt; mix on medium-low until well combined. Add the cooled chocolate and mix well; add the flour and stir until no white streaks remain. Add nuts, if using, and stir well into the batter.
Transfer the batter to the baking dish and bake about 25 -30 minutes, or until a tester, inserted in the middle, comes out clean. Remove from the oven and cool completely before cutting.