Stir together first 4 ingredients in a large bowl; cut in butter with a pastry blender until crumbly. Stir in chocolate morsels and cacao nibs.
Whisk together egg, 1 cup whipping cream, and vanilla; add to flour mixture, stirring with a fork just until dry ingredients are moistened and mixture forms a shaggy dough. Using a 1/3-cup measuring cup, scoop dough into mounds onto parchment paper-lined baking sheets. Brush scones with additional cream, and sprinkle with sugar, if desired.
Bake at 425° for 18 minutes or until golden. Serve warm.