Allow egg whites to stand at room temperature for 30 minutes. Preheat oven to 225°F. Line a large cookie sheet with parchment paper; set aside. In a large mixing bowl add cream of tartar and salt to egg whites. Beat with an electric mixer on high speed until soft peaks form (tips curl). Gradually add sugar, 1 tablespoon at a time, beating on high speed until glossy and stiff peaks form (tips stand straight). Beat in mint extract and green food coloring.
Transfer meringue to a pastry bag fitted with a 1/2- to 3/4-inch open star tip. Pipe stars 1 inch apart onto the prepared cookie sheet. Bake about 60 minutes or until meringues appear dry and are firm when lightly touched. Turn off oven. Let meringues stand in the closed oven about 60 minutes or until cool and crisp.
In a small saucepan heat and stir 2 ounces chocolate and the vegetable oil over low heat until melted and smooth. Spread about 1/2 teaspoon each melted chocolate on the flat sides (bottoms) of half the meringues. Press the flat sides of the remaining cookies against the filling. Let stand until set.
Layer the unfilled cookies between sheets of waxed paper in an airtight container; cover. Store at room temperature for up to 3 days or freeze for up to 3 months. Fill cookies before serving.