Meanwhile, to prepare filling: Place chocolate chips in a heatproof medium bowl. Combine milk and cornstarch in a medium saucepan. Bring to a boil over medium-high heat; cook, whisking constantly, until the mixture looks thick like honey and frothy, about 5 minutes. Pour it over the chocolate chips. Add peppermint extract and salt; let stand for 1 minute without stirring. After 1 minute, stir gently with a spatula until all the chocolate is melted and the mixture is thick like pudding. Press a piece of plastic wrap directly on the surface. Set aside at room temperature for 20 minutes.