Preheat oven to 350 degrees. Line 20 cups of two 12-cup muffin tins with baking cups. Sift flour, baking powder, baking soda, and 1/2 teaspoon salt into a bowl. Stir together milk and 1/4 cup orange juice.

Beat butter, 1 1/4 cups sugar, and the orange zest with a mixer on medium speed until pale and fluffy, about 4 minutes. Beat in eggs, 1 at a time. Reduce speed to low, and add flour mixture in 3 additions, alternating with milk mixture, beginning and ending with flour. Beat in poppy seeds.

Divide batter among muffin tins (filling each cup two-thirds full). Bake until golden around the edges, 18 to 20 minutes.

Meanwhile, bring remaining 1/4 cup orange juice and remaining 1/4 cup sugar to a boil in a small saucepan over medium heat, stirring until sugar dissolves. Remove from heat. Let stand until ready to use.

Brush warm cupcakes with half the orange juice mixture. Transfer cupcakes to a wire rack, and let cool for 30 minutes.

Brush cupcakes again with remaining orange juice mixture, and let cool completely.

Frost cupcakes with untinted buttercream. Top each with a buttercream poppy.