Microwave the potatoes according to package instructions.
In a small saucepan, heat 2 tablespoons butter with heavy cream and thyme sprigs. Bring to a simmer, then remove from the heat. Remove the thyme sprigs and discard. Set mixture aside.
In a small bowl combine the remaining 4 tablespoons butter, chives, and season with a salt and pepper. Mix until well incorporated.
Mash the potatoes lightly with a potato masher. Add in the cream mixture, horseradish and continuing to mash until desired consistency. Do not over mix or the potatoes will become gummy. Season the potatoes with salt and pepper. Transfer to a serving bowl and top with chive butter.