Preheat your oven to 350°F (177°C). Line a standard 12-cup muffin tin with paper liners, and line an additional pan with 2 more liners, as this recipe yields approximately 14 cupcakes. Set the pans aside.

To make the cupcakes: In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt. Set this dry mixture aside. In a large bowl using either a handheld mixer or a stand mixer fitted with the paddle attachment, beat the softened butter on high speed for about 1 minute, until it becomes smooth and creamy. Add the granulated sugar and continue beating on high speed for another 2 minutes, until the mixture is well creamed. Scrape down the sides and bottom of the bowl with a rubber spatula as needed. Add the egg whites, vanilla extract, and vanilla bean seeds (if using), then beat on medium-high speed until everything is fully incorporated. Mix in the sour cream and scrape down the bowl again as needed. With the mixer running on low speed, gradually add the dry ingredients and mix until just combined. Keeping the mixer on low, slowly pour in the milk and mix until the batter is smooth—avoid overmixing. If any lumps remain, gently whisk the batter by hand to ensure it’s evenly mixed. The final batter will be slightly thick.

Evenly divide the batter among the cupcake liners, filling each about two-thirds full to prevent overflow during baking. Bake the cupcakes for 19 to 22 minutes, or until a toothpick inserted in the center comes out clean. If you're making mini cupcakes (about 30), bake at the same temperature for 11 to 13 minutes. Once done, remove from the oven and allow the cupcakes to cool completely before adding the filling and frosting.

To fill the cupcakes: Use a sharp knife to carve a small circular hole in the center of each cooled cupcake, creating a pocket roughly ¾ inch deep. Spoon about 1 teaspoon of your favorite jelly into the center of each cupcake, then place the removed piece of cake back on top to cover the filling. If needed, you can refer to a visual guide for this step, such as a "How to Fill Cupcakes" tutorial.

To make the frosting: In a large mixing bowl using a handheld or stand mixer fitted with the whisk attachment, beat the softened butter on medium speed for about 1 minute, until smooth and creamy. Add the creamy peanut butter and beat on medium-high speed until the mixture is fully combined and fluffy. Add the confectioners’ sugar, heavy cream, vanilla extract, and salt. Begin mixing on low speed for about 20 seconds to prevent splattering, then gradually increase to high speed and beat for 1 minute, or until the frosting is smooth, creamy, and light. Frost the cupcakes using a piping bag fitted with a round tip—I used Wilton tip #12 for a clean, classic finish.