Preheat oven to 350°F. Line an 8-inch or 9-inch square baking pan with aluminum foil, making sure to leave enough overhang on the sides. Set aside.
In a large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, cream the softened butter for about 1 minute on medium speed. Once smooth, add the brown sugar and granulated sugar on medium speed until fluffy and light. Beat in the molasses. Beat in the egg and vanilla extract. Scrape down the sides as needed.
In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, cinnamon, ginger, cloves and salt. With the mixer running on low speed, slowly add the dry ingredients to the wet ingredients in 3 different additions until incorporated
Press the cookie dough evenly into the prepared baking pan. Bake for 25-27 minutes or until a toothpick inserted into the centre comes out clean. You want the cookie bars to be extra soft, so careful not to overbake. Allow the bars to cool completely on a wire rack at room temperature before frosting.