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Preheat oven to 350 degrees F.
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Combine dry ingredients (flour through salt) in a medium bowl.
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In a large bowl, beat butter and sugar until light and fluffy.
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Add eggs and then vanilla and beat until thoroughly mixed.
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Add the dry ingredients and stir until well blended.
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Mix in the chocolate chunks.
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Drop by rounded tablespoons onto ungreased cookie sheets.
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Bake for 10 minutes in the preheated oven, a minute or so longer than just set.
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Cool for one minute on the cookie sheets before transferring to wire racks to cool completely.
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To make the buttercream, beat butter and 1 cup confectioners' sugar together in the bowl of a stand mixer fitted with the paddle attachment.
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Add half and half and vanilla extract.
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Add up to one more cup of confectioner's sugar, beating after every addition until buttercream is smooth but still thick.
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To assemble the cookies, spoon frosting into a Ziplock bag. Twist closed and snip one corner with scissors, squeezing frosting to make a piping bag.
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Pipe a mound of buttercream onto one cookie and sandwich with another cookie.
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Repeat for the remaining cookies.
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Roll sides of sandwich in sprinkles.
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Pipe two dots of frosting on each sandwich.
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Place two M&Ms on top of frosting and press to adhere.
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Press two candy corns onto the frosting in the middle to make fangs.
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Refrigerate or keep in an airtight container at room temperature for up to 3 days.