Preheat the oven to 350°F (175°C) and line 2 12-cup muffin pans with 20 cupcake liners.
In a medium bowl, whisk together the flour, baking powder, cinnamon, ginger, nutmeg, and salt.
In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Add the egg yolks, pumpkin purée, Greek yogurt, sourdough discard, and vanilla, mixing until well combined.
Gently fold the dry ingredients into the wet ingredients until just combined, being careful not to overmix.
Divide the batter evenly among the cupcake liners, filling each about ¾ full.
Bake for 25 minutes, or until a toothpick inserted in the center comes out clean. Allow the cupcakes to cool completely on a wire rack before frosting.
To make the frosting, beat the softened butter and cream cheese together until smooth and creamy. Mix in the vanilla, then gradually add the powdered sugar, beating until light and fluffy.
Spread or pipe the frosting onto the cooled cupcakes and enjoy!