Preheat oven to 350°F (180°C). Break off 9 squares from chocolate bar (about 2/ 3 of bar, reserve remaining squares to top cookies) and place in mini-food processor, processing until finely chopped (or chop by hand). Transfer to a bowl and combine with dry ingredients, sifting in flour, cocoa powder, baking powder, and baking soda, and stir until well combined. In a separate bowl, combine syrup, agave nectar, vanilla and mint extracts, and oil and stir until well mixed. Add wet mixture to dry, and stir until just well combined (do not overmix). Line a baking sheet with parchment paper. Spoon batter onto baking sheet, spacing evenly. Break up or roughly chop reserved chocolate and place on top of each cookie, pressing in a little. Bake for 11 minutes (no longer, or they will dry out). Remove from oven, and let cool for 1 minute (no longer) on the sheet, then transfer to a cooling rack.