In a mixing bowl, combine the butter, sugar, and orange zest, beat using an electric mixer until fluffy.
Add in the eggs, beat again.
In a smaller bowl, combine the flour, baking powder and salt, gently mix with a spoon. Gradually add the flour mixture to the wet ingredients, mixing with a spatula.
Add in the poppy seeds, mix again by hand.
Bake for 15-18 minutes to until a toothpick inserted in the center of the cupcakes comes out clean.
Allow the cupcakes to cool for 5 minutes before removing them from the pan. Allow them to cool completely before frosting.
Buttercream
In a mixing bowl, combine the butter and vegetable shortening, beat using an electric mixer until fluffy.
Add in the fresh orange juice and zest, gently mix by hand with a spatula.
Begin to add the powdered sugar, about 2-3 cups at a time. Continue adding powdered sugar until the frosting no longer looks wet and tastes good to you.
Add a little bit of orange gel food coloring and mix one last time.
Transfer the buttercream into a piping bag fit with a Wilton 1M tip. Pipe buttercream swirls on each cupcake. Top each cupcake with an orange slice.