Preheat the oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners. Line a second pan with 2 liners—this recipe makes about 14 cupcakes. Set aside.

Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter on high speed until smooth and creamy—about 1 minute. Add the sugar and beat on high speed for 2 minutes until creamed together. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Add the egg whites, vanilla extract, and vanilla bean (if using). Beat on medium-high speed until combined, then beat in the sour cream. Scrape down the sides and up the bottom of the bowl as needed. With the mixer on low speed, add the dry ingredients until just incorporated. With the mixer still running on low, slowly pour in the milk until combined. Do not overmix. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. The batter will be slightly thick.

Pour/spoon the batter into the liners—fill only 2/3 full to avoid spilling over the sides. Bake for 19-22 minutes, or until a toothpick inserted in the center comes out clean. For around 30 mini cupcakes, bake for about 11-13 minutes, same oven temperature. Allow the cupcakes to cool completely before filling and frosting.

Using a sharp knife, cut a circle/hole into the center of the cupcake to create a little pocket about 3/4 inch deep. Place about 1 teaspoon of jelly inside and top with the piece of cupcake you removed to seal it. See my How to Fill Cupcakes post if you need extra visuals or help with this step.

Using a handheld or stand mixer fitted with a whisk attachment, beat the softened butter on medium speed for 1 minute until completely smooth and creamy. Add the peanut butter and beat on medium-high speed until the two are combined. Add the confectioners’ sugar, cream, vanilla extract, and salt. Beat on low speed for about 20 seconds, then gradually increase to high speed. Beat on high for 1 minute. Frost the cupcakes. I used a Wilton piping tip #12 to frost.