Preheat oven to 350ºF. Grease or paper-line 18 muffin tin cups.
Combine flour, baking powder, baking soda, and salt in a medium-sized bowl and whisk briefly. Set aside.
In the bowl of a standing electric mixer, beat together butter, sugar, and orange zest until light and creamy. Add eggs, one at a time, beating until just incorporated. Whisk in sour cream, orange juice, and kirsch (if using). Stir in dry ingredients, mixing until just combined.
Scoop the thick batter into the prepared muffin cups, filling each one about halfway. Bake until pale golden and a skewer inserted in the center of a cupcake comes out mostly clean, about 12 to 15 minutes.
Turn out cupcakes on a rack and cool completely before frosting.
For the buttercream, beat the butter in the bowl of a standing electric mixer until light and creamy. Add 3 cups of the confectioners' sugar, mix slowly with the paddle attachment to combine, and then increase speed to medium-high and beat until well combined. Add vanilla extract, salt, and 1/4 cup heavy cream. Beat well, adding additional powdered sugar or cream, as needed, to achieve a smooth, creamy consistency that will hold up on your cupcakes.
Pipe frosting atop cooled cupcakes or use a small spatula to add a generous layer of frosting to each cupcake. Garnish with sprinkles or orange wedges, if desired.