Put the potatoes in a large pot and cover with cold water by 1 1/2 inches. Add 2 teaspoons salt, the garlic and bay leaves and bring to a boil over high heat. Reduce the heat to medium to maintain a steady simmer and cook until the potatoes are tender but not falling apart, about 20 minutes. Drain well and return to the pot off the heat; discard the bay leaves.
Add the half-and-half, 2 tablespoons butter and a few grinds of pepper to the pot. Mash the potatoes until very smooth. Add the mascarpone and mash until creamy, adding more half-and-half as needed to loosen. Season with salt and pepper. Transfer to a serving dish.
Melt the remaining 2 tablespoons butter in a small skillet over medium heat; cook, swirling the pan occasionally, until the butter starts browning, 3 minutes. Add the sage leaves and cook until crisp, 30 seconds to 1 minute. Add a pinch of salt and swirl to dissolve; pour over the potatoes before serving.