Preheat oven to 350 degrees F.
Sift the cake flour, cocoa powder, and salt together; set aside. In a clean bowl of an electric mixer, beat the egg whites at medium-high speed until foamy. Once foamy, add the sugar substitute, a little at a time, and continue beating on medium-high speed until the egg whites start to form soft peaks. When they have formed soft peaks, fold in the vanilla extract. Remove bowl from mixer and remove beater or whisk attachment. Gently fold the flour mixture into the egg white mixture, 2 tablespoons at a time, until well blended.
Gently scoop batter into a 10-inch tube pan (angel food cake pan). Bake for 35 minutes or until firm to the touch. Remove from oven, invert, and let cool upside down in pan.
While cake is cooling, in a small bowl, combine the strawberries, yogurt, and honey until well combined. Refrigerate until ready to serve. Once cake is cool, remove from pan and slice into 8 wedges. Divide strawberry mixture evenly among the 8 wedges. Garnish each wedge with a mint sprig.