Preheat oven to 325°. Line two 12-cup muffin pans.
In a medium mixing bowl, combine flour, baking powder, salt, and poppy seeds.
In a separate large bowl, beat butter, granulated sugar, and lemon zest until light and fluffy.
Add the eggs one at a time, beating well after each addition.
Add half the flour mixture and beat on low speed just until blended.
Add milk and lemon juice and mix on low speed until combined.
Divide batter evenly among the muffin cups, filling 3/4 full. Bake until golden brown, 20-25 minutes.
Remove from the oven and cool on a wire rack.
For the frosting, beat the butter until light and fluffy.
Gradually add 2 cups powdered sugar, beating well.
Slowly add in milk and lemon juice, then beat in remaining powdered sugar.
Pipe frosting on to cupcakes; top with lemon zest and poppy seeds.