Preheat oven to 350º. Spray 2 (9") round cake pans with nonstick baking spray with flour.
In a large bowl, beat butter and sugar at medium speed with an electric mixer until creamy. Add egg yolks, one at a time, beating well after each addition. Beat in peppermint extract.
In a medium bowl, combine flour, baking powder, and salt. Gradually add to butter mixture, alternately with cream, beginning and ending with flour mixture, beating just until combined after each addition.
Stir in sour cream and crushed peppermints, just until combined.
In a medium bowl, beat egg whites at high speed with an electric mixer until stiff peaks form. Gently fold into batter. Spoon batter evenly into pans.
Bake for 26-28 minutes, or until a wooden pick inserted near the center comes out clean. Let cool in pans for 10 minutes.
Remove from pans, and cool completely on wire racks. Wrap cake layers in heavy duty plastic wrap, and chill for at least 1 hour or up to 24 hours.