Mashed Potatoes with Garlic, Mascarpone, and Caramelized Leeks

Melt butter in large nonstick skillet over medium heat. Add leeks and stir to coat. Cover and cook until leeks are golden brown, stirring every 5 minutes, about 20 minutes total. Season to taste with salt and pepper. (Can be prepared 1 day ahead. Cover and refrigerate. Rewarm over medium heat before using.)
Combine 1/2 cup milk and garlic in medium saucepan. Bring to simmer over medium-low heat. Reduce heat to very low, cover, and cook until garlic is soft, about 10 minutes. Remove from heat. Using back of fork, mash milk-garlic mixture to paste. Set aside.
Cook potatoes in large pot of boiling salted water until tender, about 25 minutes. Drain well. Return potatoes to same pot; mash until almost smooth. Add mascarpone and milk-garlic mixture; mash until smooth. Season with salt and pepper. (Can be made 2 hours ahead. Let stand uncovered at room temperature. Rewarm over medium-low heat, stirring and thinning with milk by 1/4 cupfuls if dry.) Top with leeks.
*Italian cream cheese; sold at many supermarkets and at Italian markets. If unavailable, blend 3/4 cup cream cheese with 3 tablespoons whipping cream and 1 1/2 tablespoons sour cream.