• Preheat oven to 325°F (163°C). Line two 12-cup muffin pans with cupcake liners.
• In a medium bowl, whisk together flour, baking powder, salt, and poppy seeds.
• In a large mixing bowl, beat butter, granulated sugar, and lemon zest until light and fluffy.
• Add eggs one at a time, beating well after each addition.
• Add half of the flour mixture and mix on low speed until just combined.
• Add milk and lemon juice, mixing gently until incorporated.
• Mix in the remaining flour mixture until smooth. Do not overmix.
• Divide batter evenly among cupcake liners, filling each about ¾ full.
• Bake for 20–25 minutes, or until golden brown and a toothpick inserted in the center comes out clean.
• Remove from oven and cool completely on a wire rack.

To Make the Lemon Buttercream Frosting

• Beat butter until light and fluffy.
• Gradually add 2 cups powdered sugar, beating well.
• Add milk and lemon juice, mixing until smooth.
• Beat in the remaining powdered sugar until creamy and fluffy.
• Pipe frosting onto cooled cupcakes and garnish with lemon zest and extra poppy seeds if desired.