Preparation Steps

• Make the frosting first: In a stand mixer fitted with the paddle (or with a hand mixer), beat the butter on medium speed until smooth and light, about 2–3 minutes. Add half the powdered sugar, vanilla, and a pinch of salt; mix on low until combined. Beat in the milk, then add the remaining sugar. Whip on medium-high for 3–4 minutes until fluffy. Cover and set aside at cool room temperature.
• Preheat the oven to 350°F (177°C). Line 18 standard muffin cups with paper liners.
• Bloom the cocoa: In a heatproof bowl, whisk the cocoa powder, espresso powder, and cinnamon with the very hot water until smooth and glossy. Let cool until just warm, 5 minutes.
• Whisk the dry mix: In a large bowl, whisk together the gluten-free flour, baking powder, baking soda, and salt.
• Build the wet base: In another bowl, beat the eggs and granulated sugar with a hand mixer on medium-high until paler and slightly thickened, about 2 minutes. Stream in the oil and vanilla; mix just to combine, then fold in the sour cream until smooth.
• Combine: Stir the cocoa mixture into the wet base. Add the dry ingredients in two additions, mixing gently after each just until no dry streaks remain. Do not overmix.
• Rest the batter for 8–10 minutes to hydrate the gluten-free flour. This improves the crumb without overmixing.
• Fill the liners about 2/3 full using a scoop. Bake for 16–18 minutes, rotating the pan once halfway, until the tops spring back lightly and a toothpick inserted in the center comes out with a few moist crumbs.
• Cool the cupcakes in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting.
• Frost generously with the vanilla buttercream using a piping bag or an offset spatula. Add sprinkles if you like.