Preheat the oven to 350° and line 4 muffin cups with paper liners. In a medium heatproof glass bowl set over a saucepan of simmering water, combine the chocolate chips with the butter, sugar, cornstarch and vanilla. Stir until the chocolate chips are melted and smooth. Remove the bowl from the saucepan and let the chocolate mixture cool slightly. Whisk in the whole eggs and the egg yolks.