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.

In a large bowl 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 silicone spatula as needed. Add the egg whites, vanilla extract, and peppermint extract. 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 over-mix. 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 when they rise. 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 frosting.

When the cupcakes are nearly cool, melt the white chocolate for the frosting: Place chopped white chocolate in a microwave-safe bowl (I usually use a glass liquid measuring cup), and microwave in 20-second increments, stirring after every 20 seconds until completely smooth. Set it aside to cool at room temperature for 20 minutes. (Be sure the melted white chocolate has cooled for 20 minutes, so it’s still melted but not scorching hot, which would melt the butter.)

In a medium bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed for 1 minute. Switch the mixer to low speed and slowly add the confectioners’ sugar. Stir the cooled white chocolate so that it is smooth, and then add it to the butter/sugar mixture. Switch the mixer to medium speed and beat for 2 minutes until combined and creamy. Add the cream, vanilla extract, peppermint extract, and salt. Beat for 1 minute until combined. Taste. Add 1 more Tablespoon of heavy cream/milk if needed to thin out, if desired. Add another pinch of salt if frosting is too sweet.

Frost cooled cupcakes. See Note for the swirled frosting. Top with crushed candy canes and coarse sugar sprinkles, if desired.

Store leftovers in the refrigerator for up to 3–4 days.