Preheat the oven to 350°F. Coat an 8- or 9- inch square metal baking pan with cooking spray.
In a large bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
In another bowl, whisk together ½ cup of the tahini, the sugar, ⅓ cup of the honey, the eggs, olive oil, and vanilla. Pour into the dry ingredients. Mix together until a thick batter forms.
Pour the batter into the prepared pan and spread it out evenly to each corner. Bake for 16 to 18 minutes or until the edges are visibly baked through and the center is just set. Remove the pan from the oven, place on a wire rack, and cool for 15 minutes.
While the brownies bake, squeeze 1 tablespoon of lemon juice into a small bowl. (Save any remaining lemon for another use.) Add the remaining 6 tablespoons tahini, remaining 2 tablespoons honey, and 2 tablespoons water to small bowl. Whisk together until the icing is thick and smooth.
Once the brownies have cooled, spread on the icing over the top, then cut into 16 squares.