Preheat oven to 350°F. Line muffin tin with cupcake paper liners. Set aside.
In a large bowl, whisk together flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment (or using an electric hand mixer), beat together butter, oil, and peanut butter until combined. Add brown sugar and sugar. Beat until smooth. Scrape down sides of bowl as needed. Add egg. Beat until incorporated.
In a small bowl, whisk together milk, sour cream, and vanilla.
Alternate adding dry and wet ingredients. Add 1/3 of flour mixture. Mix on low speed. Add 1/2 of wet ingredients. Mix until combined. Repeat adding dry and wet, finishing with the last 1/3 of flour mixture. Scrape down sides of bowl as needed. Mix until just combined and there are no longer and dry streaks of flour.
Divide batter among 12 cupcakes, filling liners about 2/3 full. Bake for 20-22 minutes, until toothpick inserted in center of cupcake comes out clean. Cool cupcakes in tin for 5 minutes. Remove cupcakes from tin and cool to room temperature on wire racks.