Preheat the oven to 350°F (175°C) and line a muffin tin with cupcake liners.
Mix dry ingredients in a medium bowl.
Beat butter and sugar until fluffy, then add eggs, vanilla, orange zest, and juice.
Gradually add dry ingredients alternating with buttermilk. Mix until combined.
Spoon batter into cupcake liners, filling 2/3 full.
Bake for 18-20 minutes, then cool completely.
For frosting, beat butter, add powdered sugar, then orange juice, zest, and heavy cream. Frost cupcakes and garnish with orange slices.