How to Make It
In a large cast-iron skillet, cook the bacon over moderate heat, stirring, until the fat is rendered and the bacon is crisp, 7 minutes. Using a slotted spoon, transfer to a bowl; reserve for another use.
Add the onion, apple and a generous pinch of salt to the skillet and cook over moderate heat, stirring, until starting to brown, 8 minutes. Stir in the thyme, pepper, paprika, nutmeg and cinnamon and cook until fragrant, 1 minute; transfer to a food processor.
In the skillet, melt 2 tablespoons of the butter. Season the livers with salt and cook over moderately high heat, turning, until barely pink inside, 4 minutes. Add the bourbon and simmer for 1 minute. Scrape the livers and any juices into the processor; let cool.
Add the mascarpone and remaining stick of butter to the processor and puree. Press the mousse through a fine sieve and season with salt; spread in a serving bowl. Press a sheet of plastic wrap onto the surface. Refrigerate for at least 2 hours or up to 2 days; serve at room temperature.