Prick potatoes all over with a fork. Place on paper towel and microwave on high 20-25 minutes, turning over halfway through, until tender.
Slice off 1/4 of each potato, lengthwise. Let cool 5 minutes, then scoop out the flesh of the potato, leaving a 1/4" rim on all sides. After scooping out all 7 potatoes, discard one shell.
Brush potatoes inside and out with melted butter and place on a rack set in a baking sheet. Sprinkle with salt and bake 15 minutes until crisp.
In a bowl, whisk half the Boursin cheese with 1/4 c half-and-half until smooth.
In a small pan, melt remaining 3 tbsp butter. Saute garlic until fragrant and straw-colored, 3-5 minutes. Stir in Boursin mixture.
Press potato flesh through ricer or food mill. Fold in garlic-Boursin mixture, 3 tbsp chives, salt and pepper. Remove shells from oven and fill with mashed potato mixture. Top with remaining Boursin, crumbled. Bake until golden brown, 15 minutes. Sprinkle with remaining chives before serving.