Fill a large pot with generously salted water. Place potatoes in pot and bring to a boil over medium-high heat for about 12-15 minutes or until the potatoes are fork tender. Drain and let potatoes cool and dry out for about 15 minutes.
Preheat the oven to 425 degrees and line a large baking pan with parchment paper. Set aside.
Place the cooked potatoes in a large bowl; add 3 tablespoons of butter, Boursin and egg yolks (I found that mine didn't need salt, but season to taste.) Mash or mix until mostly smooth, then add milk and mix until everything is smooth and fully combined.
Scoop the potato mixture into a large piping bag fitted with an open star tip. Pipe the mixture onto the lined baking pan into desired decorative swirls.
Melt remaining 2 tablespoons of butter and gently brush over piped potato mounds, careful not to flatten the swirls.
Bake for 20 minutes or until golden brown. Serve warm, garnished with fresh herbs if desired.