Boil and mash potatoes, then cool to room temperature. Add butter to potatoes. Mix potatoes, sugar, and vanilla extract, and chill in the refrigerator for 1 hour. The mixture should be the consistency of marzipan. Divide into 16 equal pieces and roll into egg shapes. Freeze them for 30 minutes. Dip pieces into melted chocolate to cover, transfer to a rack for the chocolate to harden. Serve cold.