COOKIES: In a small bowl, whisk together flour, cornstarch, cocoa powder and salt. In a large bowl, cream together butter and sugar. With the mixer on low speed, add in the milk and extracts. Mixture will look curdled. Gradually, add flour mixture until fully incorporated. Shape dough into two logs, about 1 1/2-inches in diameter, wrap in plastic wrap and freeze for at least 1-2 hours, until dough is very firm.
Preheat oven to 375 degrees. Slice dough into 1/4-inch thick rounds ( if they are too thick, they will not be as crisp) and place on a parchment lined baking sheet. Cookies will not spread very much, so you can put them close together. Bake for 13-15 minutes, until cookies are firm at the edges. Cool cookies completely on a wire rack before dipping in chocolate.
CHOCOLATE COATING: In a microwave safe bowl, combine chocolate and butter. Cover and melt on 50% power in the microwave, stirring every 30-60 seconds, until chocolate is smooth. Chocolate should have a consistency somewhere between chocolate syrup and fudge for a thin coating.
Dip each cookie in melted chocolate, turn with a fork to coat, then transfer to a piece of parchment paper or wax paper to set up for at least 30 minutes, or until chocolate is cool and firm. Reheat chocolate as needed to keep it smooth and easy to dip into. Store in an air tight container in the freezer.