In a bowl, whisk egg, oil and sugar until together until smooth and no lumps remain. Add melted butter, merlot and vanilla and mix until combined. Add half of the dry ingredients, stirring to combine. Add the milk, then the remaining flour mixture. Mix until batter is smooth with a large spoon. Fold in chocolate. Line a muffin tin with liners and using a heaping 1/4 cup measure, add batter to each cup until 3/4 of the way full. Bake for 15-18 minutes. Let cool completely before frosting.