Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners. Line a second pan with 2-3 liners—this recipe makes 14-15 cupcakes. Set aside.

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

Using a handheld or stand mixer fitted with a whisk attachment, beat the butter and sugar together on high speed until smooth and creamed, about 2 minutes. Scrape down the sides and up the bottom of the bowl with a silicone spatula as needed. Beat in the egg whites on high speed until combined, about 2 minutes. Beat in the sour cream and vanilla extract. 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 and mix in the milk *just* until combined. Do not overmix.

Pour/spoon the batter into the liners. Fill only 2/3 full to avoid baking over the sides. Bake for 20-22 minutes, or until a toothpick inserted in the center comes out clean. For around 35 mini cupcakes, bake for about 11-13 minutes, same oven temperature. Allow the cupcakes to cool completely before filling and frosting.

Fill the cupcakes: Using a sharp knife, cut a circle/hole into the center of the cooled cupcakes to create a little pocket about 1 inch deep. The piece you removed will be the shape of a cone. Spoon Nutella inside each carved out cupcake. Use as much Nutella that will fit. Slice off the pointy end of the “cone” piece you removed so that it can fit on top of the filling. Place on top of the filling. See my How to Fill Cupcakes post if you need extra visuals or help with this step.

Make the buttercream: Using a blender or food processor, process the freeze-dried strawberries into a powdery crumb. You should have around 1/3 – 1/2 cup. Set aside. With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add the confectioners’ sugar, heavy cream, vanilla extract, and salt. Beat on low speed for 30 seconds, then increase to high speed and beat for 3 full minutes. Divide buttercream in half. Beat 1/3 cup of Nutella into half and the strawberry dust into the other half. If either frosting seems too thin for piping, add an extra Tablespoon of confectioners’ sugar. Add an extra Tablespoon of cream if frosting seems too thick.

Swirled frosting: Fill a piping bag with a piping tip (I used Wilton 1M piping tip for the pictured cupcakes). Add a spoonful of the Nutella frosting, then a spoonful of the strawberry frosting, then continue alternating spoonfuls of frosting until the piping bag is filled. Pipe frosting onto cupcakes. I piped a basic tall swirl. See #2 in this piping tips post about piping a tall swirl. (I have a video tutorial in that blog post if you need a visual.) Decorate with garnishes, if desired.

Cover and store leftovers in the refrigerator for up to 5 days.