1 Preheat oven and prepare pan: Preheat an oven to 350°F. Spray a 9 x 13-inch baking pan lightly with cooking oil.
Line the pan with parchment paper, making sure about 1-inch of paper overhangs the long side of the pan. You will use the overhang to lift the bars out of the pan later.
2 Cream the butter, sugar, and spices: In the bowl of a stand mixer fitted with the paddle attachment, place the butter, brown sugar, vanilla extract, ginger, cinnamon, nutmeg, cloves, salt, and baking soda. With the mixer on medium-high, cream the ingredients together until the mixture forms a paste and clings to the side of the bowl, about 2 minutes.
3 Scrape, add molasses and egg, and mix: Use a rubber spatula to scrape down the side of the bowl and add the molasses and egg. Mix until incorporated, about 30 seconds on medium speed.
4 Scrape down the bowl: Use a rubber spatula to scrape down the sides of the bowl again. Add the flour. Slowly mix, at low speed, until the dry ingredients are absorbed. You may need to scrape the sides of the bowl down halfway through to make sure all the ingredients are incorporated. The dough should be the consistency of thick and stiff frosting.
5 Transfer dough to the prepared pan: Scrape the dough into the prepared baking pan and spread the dough out evenly with a butter knife or small offset spatula.
6 Bake: Transfer the baking dish to the oven and bake for 16 to 20 minutes or until a toothpick inserted in the middle of the pan comes out clean.
7 Cool the bars: Remove the pan from the oven and let it cool completely on a wire rack until it is at room temperature, about 2 hours. While the bars are cooling, take the cream cheese and butter out of the fridge to come to room temperature.
8 Make the frosting: Once the bars are cool, make the frosting by placing the cream cheese and butter in a clean bowl of a stand mixer fitted with a paddle attachment. Cream together on medium-high speed until the butter and cream cheeses are well blended and there are no more lumps, about 1 minute.
Scape down the sides of the bowl and add vanilla and salt and mix for 30 seconds to incorporate. Again, scrape down the sides and add the powdered sugar. Mix on low speed until the sugar is absorbed and the frosting is thick and smooth.
9 Frost the cookies: Frost the top of the cookie bars generously.
10 Chill and cut the bars: If you want clean lines for presentation purposes, place the frosted uncut bars, uncovered, in the fridge to chill for 1 hour. Once the bars have chilled, use the parchment sling to lift them from the pan and place them on the cutting board. Use a large sharp knife and press straight down, rather than dragging the knife through the bars. Wipe the knife blade after each cut.
If you want to skip the refrigerator chill step, you can cut the bars after they’ve cooled on the counter. The lines just might not be as sharp.
11 Serve: Arrange the bars on a platter, top with sprinkles if desired, and serve. Store leftovers covered in the fridge for up to 3 days.