Preheat the oven to 350°F (176°C) and line cupcake pans with wrappers or lightly grease with cooking spray.
Whisk flour and baking powder in a small bowl.
In a large bowl, cream butter, sugars and sour cream until light and fluffy. Beat in eggs one at a time, making sure they are fully incorporated into the batter. Mix in vanilla.
Stir in the flour mixture on low speed, alternating with milk until just combined. Spoon into prepared pan, filling each cup 3/4 full. Fill any empty cups 3/4 full of water for a more even baking.
Bake for 18-20 minutes, or until an inserted toothpick comes out with a few moist crumbs. Leave the cupcakes in the pan for 3 minutes before transferring to a wire cooling rack. Let them cool completely.
Make the frosting while the cupcakes are cooling. Cream butter and Crisco until smooth. Mix in half of the powdered sugar on low speed. Then add milk, heavy cream, 2 tbsp of champagne and the remaining powdered sugar. Mix on low speed until just combined, and then beat on high speed until smooth and fluffy. Add up to 1 tbsp of champagne if needed for taste.
Decorate as desired and store in an airtight container. I used a Wilton 1M tip.