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Place the chocolate and butter in a medium heatproof bowl. Set the bowl over a pot of gently simmering water and cook, stirring occasionally, until the chocolate has melted and the mixture is smooth. Alternatively, to use the microwave, add the butter and chocolate to a medium microwave-safe bowl and heat at half power in 30-second intervals, stirring between bursts, until smooth.
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In a medium bowl, whisk the flour, cocoa, baking powder, salt, cinnamon, and cayenne until well-combined.
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In the bowl of a stand mixer fitted with the whisk attachment, combine the sugar and eggs. Whip at medium speed until the mixture is voluminous, pale, and ribbons into the bowl when the whisk is lifted, about 5 to 6 minutes. (Alternatively, work in a large bowl with an electric hand mixer fitted with whisk attachments.)
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Add the melted chocolate mixture and vanilla to the bowl, and mix until well-combined (still using the whisk attachment), about 1 minute. Scrape down the sides and the bottom of the bowl and mix again until combined.
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Switch to the flat beater attachment (or the beaters of your electric hand mixer), add the flour mixture, and mix until almost no flour remains, less than 30 seconds. Finish incorporating any dry bits with a spatula. Cover and refrigerate the dough until very firm, at least 1 1/2 to 2 hours and up to 24 hours.
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When you’re ready to bake, preheat the oven to 325°F and line two baking sheets with parchment.
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Scoop 1 1/2 tablespoon-portions (about 28g each and 1 1/2" wide) of cookie dough; a generous tablespoon cookie scoop is ideal here. Form the dough into balls by rolling it between the palms of your hands (take heed: this will leave a chocolatey film!).
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Bake the hot cocoa cookies for 8 to 9 minutes, until the cookies are puffy and matte but still quite soft and underbaked. Remove from the oven and push a halved marshmallow, cut-side down, gently into the center of each cookie to adhere the marshmallow and slightly flatten and crack the cookies’ surface.
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Return to the oven for 3 to 5 minutes, until the cookies are set and the marshmallows have puffed. If desired, carefully broil the cookies or torch the marshmallows until browned. Let the cookies cool on baking sheets before dusting lightly with cinnamon.
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Store the cooled hot cocoa cookies in an airtight container at room temperature for up to 4 days.
