Combine cake mix with hot chocolate mix. Add milk ,eggs, vegetable oil, sour cream, vanilla extract and caramel sauce. Beat on medium speed until all ingredients are well combined.
Line a cupcake pan with liners and fill each one 2/3 full.
Bake at 350°F for 12-16 minutes. Check for doneness by inserting a toothpick in the middle of the cupcake. If the toothpick comes out clean, your cupcakes are done.
Allow cupcakes to cool completely.
For the marshmallow filling
Allow butter to soften to room temperature. Combine butter and marshmallow fluff and beat on medium-high speed until well combined and fluffy.
Add powered sugar, one cup at a time. Beat on slow speed and increase to medium speed once some of the powdered sugar has been absorbed into the butter. Mixture will be very thick.
Add a pinch of salt and 3 tablespoons of heavy whipping cream. Beat on medium high speed for 3-5 minutes until filling is fluffy.
Pour marshmallow filling in a large piping bag or Ziploc bag and cut off the bottom edge of the bag.
Core out the center of your cooled cupcakes with a knife or spoon, making a small hole. Fill each cupcake with the marshmallow filling.
Drizzle caramel over top of marshmallow filling and swirl into marshmallow with a toothpick. Set aside.
For the caramel buttercream
Allow butter to come to room temperature. This is important! Do not try to rush it by microwaving it. I recommend cutting the butter into smaller sections to help it thaw quicker.
Combine sugar and egg whites in your stainless steel mixer bowl (or any heat proof bowl). Attach your candy thermometer to the side of the bowl.
Set your bowl over top of a pot with water, just enough water to cover the bottom of the pot. You do not want the water to boil and touch the bottom of your mixing bowl. Heat on medium temperature.
Stir the egg whites and sugar constantly until the mixture reaches 150° F. This is the most important part; you do not want to egg whites to cook- so keep stirring! I have read that you can heat it be anywhere between 120° F – 160° F. When it reaches 150° F, the sugar should be dissolved in the egg whites. You can test this by placing a drop on your fingertips and rubbing them together. The mixture should be completely smooth.
Place the mixing bowl on your stand. Using the wire whisk attachment beat the eggs. Start on low speed and increase the speed to medium after 2 minutes.
Beat for another 3 minutes on medium speed. At this point, your frosting should have turned white as the egg whites are whipped and the body of the meringue starts to form.
After the first 5 minutes, increase speed to high. You will notice that as you increase the speed, the body of your meringue will also increase. It will start to look glossy as the peaks begin to form. Beat for another 5 minutes.
After you have beaten the meringue for a total of about 10 minutes, the bottom of your mixing bowl should be cooled completely. Your SMB should look kind of like whipped cream. Scrape down all sides and bottom of the bowl.
Turn your mixer down to medium-low. Slowly add your softened butter, about 1 tbsp at a time, ensuring it is mixed well after each addition. Once you have added all your butter, add vanilla extract. Mix for another minute and scrape down the sides of the bowl.
Switch to the paddle attachment and continue beating on medium to allow all the air bubbles to escape. Add caramel sauce into frosting. Beat for about 2-3 minutes until the frosting is smooth. If your frosting has not formed yet, continue to beat until it is light and fluffy.
Frost the marshmallow filled cupcakes and sprinkle top with sea salt. You can drizzle with additional caramel if desired.