Preheat the oven to 180C/350F and spray a 9x13 inch pan with cooking spray. Place a strip of grease proof paper long ways and another short ways to help you lift the cookie bars out later on.
Place the melted butter and sugar into a large bowl and whisk until there are no lumps of sugar. Add in the eggs and whisk until combined.
Pour in the black treacle/molasses and whisk until the mixture comes together and goes a little stiff.
Add in the flour, bicarb, baking soda, salt and spices and use a wooden spoon to gently fold the flour into the mixture. The mixture will be thick and sticky. Stir until there are no lumps of flour.
Spoon the mixture into your prepared pan and spread into an even layer. I found it easier to use my hands for this as it's sticky and kept sticking to the wooden spoon.
Place in the oven for 15-20 minutes until it looks matte in the oven light and the edges are firm to the touch. If you wiggle the pan it should move as one, but still be soft in the centre. You can insert a skewer, it shouldn't have liquid batter on it, but a few crumbs is fine. You want to be careful not to over bake as you'll end up with a rock!
Leave to cool in the pan completely, then make the frosting: Place the cream cheese, butter and vanilla into a large bowl and beat until well combined and fluffy, around 2 minutes on medium speed with a hand held mixer.
Slowly add the icing sugar, mixing between each addition, until you have a thick, spreadable frosting. NOTE: I used 2 1/2 cups, but after spreading it, I should have used the full 3 cups. It will depend on what cream cheese you use because some are not as thick as others. Philadelphia is my favourite, I used Waitrose this time and will go back to Philly next time! It was thick, but slightly glaze like this time. Still tasty and the drips look good in photos, but I will still use Philly next time as this recipe always makes luscious, thick frosting when I use it!
Spoon the frosting onto the cooled cookie bars and spread to cover. Top with Christmas sprinkles (or any sprinkles) then slice into 12 bars.
Gingerbread Cookie Bars will keep in an airtight container, in the fridge, for 3 days. You can also keep them without the frosting, in a container at room temperature for 5 days.