Place the dark chocolate chips in a heatproof bowl.
In a small saucepan over medium heat, warm the heavy cream until it just begins to simmer. Do not let it boil.
Pour the hot cream over the chocolate and let it sit for about 4 minutes to soften the chocolate.
Pour the Irish cream liqueur into the mixture.
Using a spatula or whisk, stir the mixture in small circles at first, then gradually expand your motions until the ganache is smooth and glossy. If it needs to melt more, pop it in the microwave for 15 seconds at a time, stirring in between.
Add the room-temperature butter and continue stirring until fully melted and combined. If using unsalted butter, add a small pinch of salt.
Distribute the ganache evenly over the mini pies, I like to use a cookie scoop to do this. Place in the refrigerator until just set, about an hour.
Top each mini pie with a generous spoonful of marshmallow fluff, toast it with a kitchen torch until golden and caramelized, then enjoy immediately.