Preheat the oven to 350 F and place a rack in the center of the oven. Line a half size (13" x 18") baking sheet with a silicone baking mat or a sheet of parchment paper.
In a medium mixing bowl add in the melted butter, both sugars, vanilla, the spices, and salt. Whisk to combine and then add in both eggs. Whisk once more.
Dump in the flour, starch, baking soda and baking powder. Whisk to combine - the batter should be thick but too runny to scoop.
In a microwave safe bowl or in a double boiler on the stove, melt the chocolate. If using the microwave, melt the chocolate in 15 second bursts stirring between each one.
Use an offset spatula or a silicone spatula to spread the cookie dough over the lined baking sheet.
Swirl the melted chocolate into the cookie dough.
Bake the cookie bar for 4 minutes, then rotate and bake for another 4-5 minutes. Don't over bake this! It'll continue to bake as it cools on the baking sheet.
Once baked, take the pan out of the oven, sprinkle the top with flaked salt and leave it to cool completely before trying to move it.