Add the softened butter and granulated sugar to a mixing bowl and mix until light and creamy.
Add the egg and mix until well combined.
Add the vanilla extract and mix until smooth.
Add the pumpkin puree and mix until evenly incorporated.
In a separate bowl, combine the cake flour, baking powder, baking soda, salt, and pumpkin pie spice.
Add the dry ingredients to the wet mixture and whisk until combined.
Add the buttermilk and mix until combined.
Continue mixing until a smooth pumpkin batter forms. Divide the batter evenly among the lined cupcake cups.
Bake at 350°F (175°C) for 18–22 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cupcakes from the oven and allow them to cool completely.
For the frosting, beat the softened butter and cream cheese until smooth and creamy, then add the powdered sugar, pumpkin pie spice, and pumpkin puree and beat until smooth and fluffy.
Transfer the frosting to a piping bag fitted with a star tip and pipe it onto the cooled cupcakes.
Finish with neat swirls of frosting and serve the pumpkin cupcakes.
