In a bowl, whisk egg and sugar until smooth and no lumps remain. Add cream, butter and vanilla, and mix until combined. Stir in sour cream.
Sift dry ingredients together and add to wet mixture. Add root beer concentrate and root beer. Mix until batter is smooth.
Line a muffin tin with liners and using a heaping 1/4 cup measure, add batter to each cup. Bake for 15-18 minutes. Let cool before frosting.
Frosting
In the bowl of an electric mixer, cream butter until smooth. On low speed, add powdered sugar one cup at a time, increasing speed to bring the frosting together. Add milk and vanilla extract. Beat until smooth. Add root beer concentrate and root beer.
Once the cupcakes have completely cooled, fill each one with root beer frosting, then top with vanilla root beer frosting.
Notes
I used an apple corer to get the middles of the cupcakes out. It’s so easy – then just push back down to cover back the holes once fillings are inside.2. I used an ice cream scoop to scoop my frosting onto the cupcakes.3. I also used a simple syrup over the cupcakes to keep them very moist – equal parts water and sugar boiled together then cooled, and added about 3 tablespoons of root beer.4. These cupcakes last a very long time – I was impressed that days later they were still soft, moist and delicious.5. Do NOT refrigerate these cupcakes. I made that mistake testing one and the frosting became rock hard and the cupcake dried out. While more simple syrup did help it, it’s best to store them in an airtight container.