Fill a large pot with salted water. Add potatoes and bring to a boil over medium-high heat for about 12-15 minutes or until fork tender. Drain and let cool for about 15 minutes.
Place the cooked potatoes in a large bowl; add 3 tablespoons of butter, Boursin and egg yolks. Mash 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.