Preheat the oven to 350°. Use some olive oil to grease a 9x9-inch square baking tin.
Melt the chocolate. Microwave on low, stirring every 30 seconds until creamy. Or, on the stove, slowly melt the chocolate using a double boiler or a metal bowl placed over a pot of water.
Remove the melted chocolate from the heat and transfer it to a large mixing bowl. Add the olive oil, sugar and light brown sugar and stir with a wooden spoon to combine.
Continue mixing, adding one egg at a time. Then add the vanilla.
Fold in the dry ingredients with the wet mixture, folding in the walnuts last. Pour into baking tin and bake for 25 to 35 minutes. The brownies should come slightly away from the side and feel firm to the touch. Let cool on a rack before cutting into squares and serving.