Preheat the oven to 350F. Line an 8×8-inch pan with aluminum foil, spray with cooking spray; set aside.
To the bowl of a stand mixer fitted with the paddle attachment (or large mixing bowl and handheld electric mixer), add the brown sugar, butter, and beat until smooth. Stop and scrape down the sides of the bowl.
Add the cinnamon, ginger, nutmeg, cloves, salt, and beat to combine.
Add the flour (don’t overmeasure or hard-pack the flour or bars will be dry and crumbly), baking soda, and beat to combine. Mixture will be dry.
Add the molasses and beat to combine.
Turn mixture out into pan and hard-pack it with a spatula. It will be on the dry and crumbly side but bakes together fine.
Bake for about 20 minutes or until center is just set. Edges will be pulling away slightly from pan; don’t over-bake because bars firm up as they cool. Place pan on a wire rack to cool for about 45 minutes before adding the icing.
To a medium bowl, add the confectioners’ sugar, liquid of your choice, and whisk until smooth and combined. Play with the liquid and sugar amounts as necessary to achieve desired consistency.
Evenly drizzle icing over bars, noting you will likely have extra. Alternatively, pour icing over bars for complete coverage. Icing will set up in about 15 minutes. I recommend waiting until it sets up before slicing and serving. Bars will keep airtight at room temp for up to 5 days or in the freezer for up to 3 months.