Preparation Steps

• Line two 12-cup muffin pans with 18 paper liners and set the oven to 340°F (170°C).
• Make the blueberry filling: In a saucepan, combine blueberries, sugar, honey, lemon zest, lemon juice, and a pinch of salt. Let sit 10 minutes to macerate and draw out juices.
• Place the saucepan over medium heat and cook, stirring often, until the berries release juices and begin to break down, 6–8 minutes. Mix cornstarch with 1 tablespoon water, then stir it into the berries. Cook 1–2 minutes more until glossy and thick. If needed, loosen with the remaining 1 tablespoon water. Spread the mixture on a small sheet pan to cool quickly; once cool, blend 3 tablespoons until silky for the frosting and reserve the rest chunky for filling. Chill both until cold.
• For the cupcakes: In a large bowl, rub the lemon zest into the sugar with your fingertips until the sugar is fragrant and slightly damp. Whisk in the eggs vigorously for 60–90 seconds until lighter in color and slightly thick. Slowly stream in the oil while whisking to emulsify.
• Whisk in the sour cream, vanilla, lemon juice, and buttermilk until smooth.
• In a separate bowl, whisk together the flour, baking powder, salt, and cardamom. Sift this over the wet ingredients and fold with a spatula in two additions just until no dry streaks remain. Let the batter rest 5 minutes to hydrate.
• Portion the batter into liners, filling each about 2/3 full. Bake 17–20 minutes, rotating the pans halfway, 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.
• Make the buttercream: Beat the butter on medium-high with a mixer until pale and fluffy, about 3 minutes. Add half the powdered sugar and mix on low until combined, then beat on medium for 1 minute. Add vanilla, lemon juice, and salt; beat to combine. Add remaining powdered sugar and beat 2 minutes until smooth and spreadable. Adjust with a spoonful more sugar for thickness or a few drops of lemon juice for looseness.
• Split the frosting evenly between two bowls. To one bowl, fold in 2–3 tablespoons of the cooled blueberry puree until evenly tinted. For a clean swirl, spoon the vanilla and blueberry frostings side-by-side into opposite sides of the same large piping bag fitted with a large open star or round tip.
• Fill and finish: Use a small paring knife to cut a 3/4-inch-wide, 1-inch-deep cone from the center of each cupcake. Spoon in about 2 teaspoons of the chunky blueberry filling, trim the cone tips, and replace the caps. Pipe a tall swirl of the two-tone frosting over each cupcake.