Line an 8x8 straight-sided metal baking pan with parchment paper. Being sure the parchment is long enough to overhang the sides, making it easier to lift the chilled fudge from the pan for slicing.

In a large microwave-safe bowl add the milk chocolate chips and sweetened condensed milk. Stir to combine then top with the cold butter cubes.

Microwave on high for 1 minute 30 seconds. Allow the ingredients to sit for 30 seconds in the bowl, to allow the chips to continue to soften, before adding the Baileys Irish Cream liqueur and vanilla extract. Stir until all the ingredients are fully incorporated and the mixture is smooth. You may need to heat for an additional 20-30 seconds if needed to get a smooth Baileys fudge mixture.

Pour the fudge mixture into the prepared pan and spread to an even layer. Tap the pan on the counter a couple times to force any air bubbles to the surface.

Place the Baileys fudge into the refrigerator to chill and set for at least 4 hours or until firm enough to easily slice.

Once fully chilled and set, carefully lift the fudge from the pan using the overhanging parchment paper. Slice into 64 1-inch pieces to plate and serve.