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Preheat the oven to 325°F with a rack in the upper and lower thirds. Line 2 baking sheets with parchment.
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To make the dough: In the bowl of a stand mixer fitted with the flat beater attachment or working in a large mixing bowl with an electric hand mixer, beat the butter, sugar, salt, and espresso powder on medium speed until smooth (not light and fluffy). If the butter is at room temperature, this will take 1 to 2 minutes. If the butter is cold, this will take 2 to 3 minutes.
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Scrape the sides of the bowl, then beat in the egg and vanilla until fully combined.
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Add the flour, cocoa, cornstarch, and baking soda all at once and mix until thoroughly incorporated. Scrape the bowl and mix again briefly.
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Scoop the dough by the tablespoonful (about 24g each) and roll into balls; a level tablespoon cookie scoop works well here.
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Place the balls of dough on the prepared baking sheets, leaving 1 1/2" between them.
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Bake the cookies for 28 to 32 minutes, rotating the pans from top to bottom and back to front halfway through. The cookies are done when the edges and centers are firm and crisp; avoid soft centers.
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Remove the cookies from the oven and allow them to cool completely before decorating, about 45 minutes.
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To make the glaze: In a very small microwave-safe bowl, combine the entire packet of natural orange food coloring with the vanilla and 2 teaspoons water. Stir together and microwave in 5-second intervals until the color has dissolved and the mixture is smooth.
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In a separate small bowl, place the confectioners’ sugar then pour the orange liquid over the top; stir to combine. Add the remaining 3 teaspoons water, 1/2 teaspoon at a time, until the glaze has a thin, paint-like consistency (see “tips,” below for instructions on how to make a pipeable glaze).
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Use a pastry brush to apply a thin coating of glaze to every cookie. Allow the cookies to rest at room temperature, undisturbed, for 10 to 15 minutes until set.
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Storage information: Store leftover Halloween cookies, covered, at room temperature for several days. Freeze unglazed cookies for longer storage; freezing glazed cookies is not recommended.
