Preheat oven to 350°.
Lightly spoon flour into a dry measuring cup, and level with a knife. Combine flour, cocoa, baking soda, and salt; stir with a whisk.
Place granulated sugar and butter in a large bowl; beat with a mixer at medium speed until well combined (about 3 minutes). Add egg substitute and vanilla, beating well. Add flour mixture and buttermilk alternately to granulated sugar mixture, beginning and ending with flour mixture. Fold in chocolate. Spoon batter into 12 muffin cups lined with muffin cup liners. Bake at 350° for 18 minutes or until cake springs back when touched lightly in center or until wooden pick inserted in center comes out clean. Remove from pan; cool completely on a wire rack. Sprinkle with powdered sugar just before serving.