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1
Preheat the oven to 400u00b0F. Lightly grease (or line with parchment) two baking sheets.
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2
Make the cookies: In a large mixing bowl, cream together the butter, salt, vanilla, baking powder, and sugar. Add the eggs one at a time, beating well after each addition. Add the flour to the wet ingredients alternately with the milk, beginning and ending with the flour. Add the green food coloring and mix until incorporated. Do this gently; there's no need to beat the batter. Stir in the chocolate chips at the end.
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3
Using an ice cream scoop, drop the dough onto the prepared baking sheets. Leave 2 to 2 1/2 inches between each cookie, for expansion.
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4
Bake the cookies for 11 minutes, or until they're barely set on top; if you see them browning on the bottom, they've baked too long. Cool them on the pan for 5 minutes, then transfer to a rack to cool completely.
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5
To make the filling: Place the chocolate chips, corn syrup, butter, and heavy cream into a large microwave-safe bowl. Heat in the microwave for 2 to 3 minutes, until the mixture is very hot. Stir until the chocolate melts and the mixture is smooth. Add in the peppermint and stir one more time. Chill in the refrigerator, stirring occasionally, till the filling is cool room temperature. Remove from the fridge, and beat at high speed with an electric mixer until it lightens in color and thickens slightly. If the filling doesn't lighten and thicken after several minutes of beating, chill for another 15 minutes, then beat again. Spread a generous amount of filling on one of the flat sides of the cookies; sandwich another half on top to form your pie. Repeat with the remaining pies. Store in an airtight container--it's better not to freeze these. Because the tops of the whoopie pies can be sticky, I'd suggest individually wrapping in waxed paper rather than plastic wrap.
