1. Heat & Prep
   • Preheat your oven to 350°F (175°C).
   • Line two 6-cup muffin tins (or one 12-cup pan) with paper liners.
2. Dry Mix
   • In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
3. Cream Butter & Sugar
   • In a large bowl, beat the butter, sugar, and lemon zest on medium speed until light and fluffy (about 2–3 minutes).
4. Add Eggs & Vanilla
   • Beat in eggs one at a time, then blend in vanilla until just combined.
5. Alternate Wet & Dry
   • With the mixer on low, add half the sour cream, then half the flour mixture. Scrape the bowl, then repeat with the remaining sour cream and flour mix.
6. Lemon & Poppy Seeds
   • Gently fold in lemon juice and poppy seeds—do not overmix.
7. Fill & Bake
   • Spoon batter into liners, filling each about two-thirds full (you’ll get roughly 14 standard cupcakes).
   • Bake for 16–20 minutes, until tops spring back and a toothpick comes out clean.
   • Let rest in the pan for 10 minutes before transferring to a wire rack to cool completely.

Blackberry Frosting

1. Purée & Reduce
   • In a blender or food processor, purée blackberries until smooth. Push through a fine-mesh sieve, using a spoon to extract about 4 Tbsp of seed-free purée.
   • Simmer the purée in a small saucepan over low heat for 10–15 minutes, stirring occasionally, until thickened like jam. Cool completely.
2. Whip Butter
   • In a mixing bowl, beat the softened butter until creamy.
3. Build the Frosting
   • On low speed, add 2 cups of the powdered sugar and the pinch of salt, then the cooled blackberry reduction.
   • Gradually beat in the remaining sugar, adding a splash of cream if the frosting feels too stiff, until you reach a pipeable consistency.
4. Decorate
   • Transfer the buttercream to a piping bag fitted with your favorite tip (a large star works beautifully) and swirl atop each cooled cupcake.