Melt chocolate and butter in a large bowl over a double boiler. Stir until all the chocolate has melted and the mixture is smooth.
Add the molasses, sugar and cocoa and stir until fully combined.
Add the eggs, whiskey, vanilla, flour, and salt; mix until just combined. Do not overmix.
Prepare a 9×13″ glass or ceramic baking dish with cooking spray and pour batter into pan.
Bake at 350 degrees for 35 minutes. Do not overbake.
Remove from the oven and cool.
Make the buttercream by combining butter, powdered sugar, Skrewball whiskey and peanut butter in a stand mixer and whip on high with a paddle attachment for 5 minutes.
Frost with buttercream, slice and serve.