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Set it to 350ºF (175ºC). Cream the butter, adding sugar slowly. Combine until uniform and lighter in color. Add the flour in two or three increments, mixing in each one until well combined. Thoroughly mix in baking soda, salt, cocoa powder, and vanilla extract. Knead the dough until it sticks together. Chill for 15 minutes. Roll out the dough to about ¼ in (0.6cm) thickness. Cut out cookies using cookie cutters in any shape. Transfer to a parchment-paper lined baking sheet. Bake until fragrant and just beginning to crack on the surface. This can take anywhere from 8 to 15 minutes, depending on how large the cookies are. Check on them regularly. Let them cool completely on a wire rack or parchment paper. This takes about 5–10 minutes. You can melt the chocolate chips in a microwave, but they may burn or separate. For best results, melt over a double boiler. Stir in the peppermint oil. You can use a pastry brush or the bottom of a spoon to help spread it evenly. Alternatively, dip the cookies into the chocolate. Let cool completely on a wire rack, with something to catch the dripping chocolate.
Preheat your oven. Mix the butter and sugar. Mix in remaining cookie ingredients. Knead and chill. Form the cookies. Bake the cookies. Let cool. Melt the chocolate chips and peppermint oil. Pour or spread the chocolate onto the cooled cookies.