Preheat the oven to 350 degrees F. Line a 8x8inch brownie pan with parchment paper, allowing excess paper to hang over all 4 of the sides (this will make it easier to remove the brownies). Lightly spray the parchment paper with cooking spray.
In a medium bowl set over a pot of simmering water, warm the olive oil and bittersweet chocolate together over low heat, stirring until the chocolate is completely melted and smooth.
Remove from the heat and stir in the sugar and vanilla, then beat the eggs in one at a time. Add the flour, espresso powder and vigorously stir for 1 minute until the batter becomes smooth and shiny.
Scrape the batter into the prepared brownie pan and sprinkle some fleur de sel evenly on top of the batter. Place in the oven and bake for approximately 30 minutes.
Let cool completely before removing and cutting the brownies.
You want to use an olive oil that's mild in flavor as opposed to one with with aggressive notes (peppery, grassy, etc). Also, keep in mind that olive oils can go rancid with time. Try to use only the freshest olive oil with this recipe (and please taste it before using).Adapted from Ready For Dessert by David Lebovitz