Preheat oven 350 degrees. Line muffin tins with liners (24 total)
CAKE:
Put all ingredients except poppy seeds into mixer and mix until completely combined, about 1-2 minutes.
Fold in the poppy seeds.
Pour batter into cupcake liners, about 1/2 full.
Bake 19-23 minutes or until toothpick inserted comes out clean.
BUTTERCREAM:
Mix butter on medium until light and fluffy.
With mixer on low add powdered sugar one spoonful at a time adding lemon juice and milk as needed. Mix until smooth and creamy.
Add in the zest and and mix until combined.
ASSEMBLY:
Take cupcakes out of muffin tray(s).
Put buttercream into piping bag fitted with tip of our choice (I used Wilton 1M).
Pipe frosting onto cupcakes and decorate.