Preheat oven to 350°F (175°C) and line a 9x13 paper with parchment paper (use enough so the parchment is hanging over the sides so that you will be able to lift it out of the pan once it's baked and cooled) or lightly grease and flour.
Stir together melted butter, brown sugar, and molasses in a large bowl.
Add egg and vanilla extract and stir well. Set aside.
In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, salt, ginger, cinnamon, cloves, and nutmeg.
Gradually add flour mixture to wet ingredients and stir until completely combined.
Spread batter evenly into prepared baking pan. Transfer to 350°F (175°C) and bake for 22 minutes. Allow to cool completely before decorating with frosting.
Once your cookie bars have cooled, prepare your frosting by combining butter, cream cheese, and vanilla extract. Use an electric mixer (or a stand mixer) to cream ingredients together until well-combined.
Gradually (with mixer on low-speed) add powdered sugar until completely combined. Stir in cornstarch (make sure it's well incorporated into the frosting).
Spread or pipe (I used a Wilton 2B tip) frosting over completely cooled gingerbread bars. Decorate with sprinkles (if desired), then slice and serve.
*Not a fan of cream cheese frosting? Try the simple buttercream frosting that I use on my Sugar Cookie Bars, instead!** Corn starch helps the frosting to harden or "crust" without affecting the flavor You can leave this out but your frosting will be softer.