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Lightly grease a baking sheet or line with parchment.
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In a small bowl or 2-cup liquid measuring cup, combine the cocoa powder, water, vanilla, and espresso powder. Stir until smooth, then place the bowl in the refrigerator.
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Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess.
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In a medium bowl, whisk together the flour, sugar, baking powder, salt, and baking soda.
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Using your fingers, a fork, pastry blender, stand mixer, or food processor, work the butter into the flour mixture until incorporated; your goal is an evenly crumbly mixture (think breadcrumbs).
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Add the mini chips or chopped chocolate and mix until evenly distributed.
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Remove the cocoa powder mixture from the refrigerator and whisk in the buttermilk.
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Add the chocolate buttermilk mixture to the dry ingredients and use a flexible spatula to stir very gently, just until combined. The dough is more moist than traditional biscuit dough; it should be wet and scoopable, like soft cookie dough.
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Use a muffin scoop or 1/4-cup measure to scoop the dough onto the prepared sheet in scant 1/4-cupfuls, leaving about 2" between each.
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Sprinkle the tops of the chocolate biscuits with sparkling sugar.
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For the best texture and highest rise, place the pan of biscuits in the freezer for 30 minutes, uncovered. While the biscuits are chilling, preheat the oven to 425°F with a rack in the top third.
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Remove the chocolate biscuits from the freezer and bake for 14 to 18 minutes, until set and the whole kitchen smells like chocolate. Serve warm or at room temperature; splitting in half and serving as shortcakes with macerated berries and whipped cream is one of our favorite ways to enjoy these chocolate biscuits.
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Store leftover chocolate biscuits, well wrapped, at room temperature for several days; freeze for longer storage.