Preparation Steps

• Reduce the strawberries: Blend or mash the diced strawberries, then cook the puree with the lemon zest in a small saucepan over medium heat, stirring often, until thickened and reduced to about 1/3 cup, 6–8 minutes. Scrape into a shallow bowl and chill until completely cool.
• Preheat the oven to 340°F (170°C). Line a muffin tin with 16 paper liners.
• Bloom the cocoa: In a medium bowl, whisk the cocoa powder with the very hot coffee until smooth and glossy. Let stand 5 minutes to cool slightly.
• Whisk the wet ingredients: In a large bowl, whisk together the sugar, vegetable oil, eggs, egg yolk, and vanilla until emulsified and slightly lightened, 1 minute. Whisk in the Greek yogurt until silky.
• Combine the dry ingredients: In a separate bowl, whisk the flour, baking powder, baking soda, and salt until evenly combined.
• Make the batter: Stir half of the dry ingredients into the wet mixture. Add all of the cocoa–coffee mixture and whisk just to combine. Fold in the remaining dry ingredients with a spatula until no flour streaks remain. Let the batter rest 5–10 minutes to hydrate for taller domes.
• Bake: Fill each liner about 2/3 full. Bake 18–20 minutes, or until the tops spring back and a toothpick comes out with a few moist crumbs. Cool in the pan 5 minutes, then transfer to a rack to cool completely before frosting.
• Start the Swiss meringue: Place the egg whites and granulated sugar in a clean heatproof mixing bowl set over a pot of barely simmering water (the bowl should not touch the water). Whisk constantly until the mixture reaches 160°F and no sugar granules are felt when rubbed between two fingers, 3–5 minutes.
• Whip the meringue: Transfer the bowl to a stand mixer fitted with the whisk. Beat on medium-high until glossy, medium-stiff peaks form and the bowl is cool to the touch, 8–10 minutes.
• Finish the buttercream: With the mixer on medium-low, add the butter a tablespoon at a time until fully incorporated and smooth. Mix in the vanilla. Beat in 2–3 tablespoons of the cooled strawberry reduction and the strawberry jam, adjusting to taste and stopping if the texture softens too much. Switch to the paddle attachment and beat 30–60 seconds to smooth out air bubbles.
• Assemble: If desired, core a small plug from each cupcake and spoon in 1/2 teaspoon of the remaining strawberry reduction. Pipe the strawberry buttercream on top with a large open star tip. Garnish with a fresh strawberry slice just before serving.