Line two small foil trays or loaf pans with aluminum foil, leaving enough foil hanging over the sides to lift out the fudge later, and butter the foil lightly.

Set a small or medium slow cooker to LOW and add the chocolate, both cans of sweetened condensed milk, and the 2 tablespoons of butter.

Cover the slow cooker and warm the mixture on LOW for about 45 minutes, leaving it undisturbed for the first 20 minutes so the chocolate can soften gently.

Remove the lid and stir slowly but thoroughly with a heatproof spatula, scraping the bottom and sides of the crock; if firm chocolate pieces remain, cover and continue on LOW in 10-minute increments until completely smooth.

Turn off the slow cooker or switch it to WARM, leave the fudge uncovered for 5 to 10 minutes, and stir once or twice while it cools slightly.

Stir in the Irish whiskey steadily until fully incorporated, continuing to scrape the corners until the mixture becomes glossy and smooth again.

Immediately scrape the fudge into the prepared foil-lined pans, divide it evenly, spread it into the corners, and smooth the top with a spatula.

Let the pans sit at room temperature until the fudge cools and firms, about 2 to 3 hours depending on the warmth of the kitchen.

Refrigerate the pans for at least 1 more hour for cleaner cuts and a dense, creamy texture.

Lift the set fudge from the pans using the foil overhang, place it on a cutting board, peel back the foil, and cut the fudge into thick, small squares with a sharp knife, wiping the knife with a warm, damp cloth between cuts.