Preheat the oven to 180°C / 350°F / Gas mark 4.
Line a 20 hole muffin tin with paper cases.
In a small bowl, whisk together the cocoa powder and hot water until smooth.
In a medium bowl, whisk together the flour, bicarbonate of soda, baking powder, and salt then set aside.
In a medium saucepan, combine the butter and the sugar over medium heat. Stir occasionally until the butter is melted.
Remove the mixture from the heat and transfer to the bowl of an electric mixer fitted with a paddle attachment.
Beat on medium-low speed, 4-5 minutes, until the mixture is cooled, alternately a food processor can be used or done by hand.
Mix in the eggs and egg yolk, one at a time, scraping down the sides of the bowl as needed and beating well after each addition.
Mix in the vanilla and then the cocoa mixture and mix well until incorporated.
With the mixer on low speed add in the dry ingredients in two batches, alternating with the sour cream, beating just until combined.
Divide the batter between the prepared cupcake liners, filling them about ¾ of the way full.
Bake for 18-20 minutes or until a toothpick inserted in the centre comes out clean.
Allow the cupcakes to cool in the tins for 5-10 minutes, then transfer to a wire rack to cool completely.
To make the caramel sauce, melt the butter in a heavy-bottomed saucepan over medium heat.
Add the sugar and cook, stirring occasionally, until the sugar begins to foam a bit.
Remove from the heat and add the double cream.
Stir until the sauce is smooth, then mix in the vanilla and salt. Let cool.
To fill the cupcakes, cut a cone out of the centre of each cupcake with a paring knife.
To make the filling, combine the chopped Snickers bars in a bowl with a few table spoons of of the caramel sauce, and mix to coat.
Drop a spoonful of the filling mixture into each cupcake.
To make the frosting, use either an electric mixer or food processor then add the butter.
Beat or blitz for 1 minute until smooth.
Blend in the confectioners’ sugar until smooth.
Mix in the vanilla, caramel sauce and salt until well combined.
Add the cream and whip on high speed until light and fluffy, about 3-4 minutes.
Transfer the frosting to a pastry bag or a plastic bag with the tip cut off and frost the cupcakes.
Use the additional caramel sauce and garnish with chopped Snickers bars bits if you like.