You can use a sugar substitute like Swerve or Monkfruit.
You can mix the ingredients directly in the jar or in a bowl first.
For a layered jar look, add sugar, flour, baking powder, and salt first and tamp down, then add cocoa and tamp down, then top with chocolate chunks.
To bake the brownies, preheat oven to 350 degrees, spray an 8x8 inch pan, combine brownie mix with 1/2 cup melted butter, 2 large eggs, and 1 teaspoon vanilla extract, then bake for 20-22 minutes.