How to make the chocolate filling:
Place the Butlers Milk Chocolate Bar in a medium sized heatproof bowl and set aside
Heat double cream in a small saucepan on medium high heat just until cream starts to bubble around the edges. Remove from heat and pour cream over the chocolate. Stir with a whisk or fork until all of the chocolate has melted and the mixture is smooth. If the chocolate stops melting, place the bowl over a pot of barely simmering water and continue stirring until the heat from the steam has melted all of the chocolate and the mixture is smooth.
Leave at room temperature for about 30 minutes to cool, and then place in the fridge and leave to chill while you make the cupcakes.
How to make the cupcakes:
For the cupcakes, preheat the oven to 180°C/160°C fan assisted/gas 4.
Line a muffin tin with 12 paper cupcake liners.
Sieve together the flour, cocoa powder, baking powder and bicarbonate of soda in a large bowl. Whisk in caster sugar and salt.
In a medium sized jug, whisk together the wet ingredients except the boiling water. Add to the dry ingredients and mix with a whisk or electric mixer just until mixture is smooth. Slowly add the boiling water and mix until well combined. The batter will be very liquid.
Pour batter evenly between cupcake liners and bake in preheated oven until cupcakes spring back when lightly pressed or toothpick inserted comes out clean, about 18-22 minutes. Remove from oven and leave to cool for about 10 minutes on a wire rack, then remove cupcakes from muffin tin and leave cakes to cool completely.
How to make the frosting:
To make the frosting, melt the Butlers 70% dark chocolate in a heatproof bowl set over a pot of simmering water, or in short bursts in the microwave. Leave to cool for about 10 minutes.
In a large bowl, beat butter with an electric mixer until smooth. Add icing sugar, milk or cream, vanilla and salt. Start beating slowly with a mixer on lowest speed; once icing sugar has been mixed in, increase speed and beat until frosting is smooth and creamy. Add the melted chocolate and beat in on low speed until frosting is a uniform colour. Make sure to scrape down the sides of the bowl so that everything is well incorporated.
To finish the cupcakes, use an apple corer to remove the centres of each cupcake. If you don't have an apple corer, you can use a thin bladed sharp knife to cut out a small bit of the centre.
If the chocolate filling is too firm to spoon or pipe, melt it very slightly in the microwave on a low setting for short bursts. Once it is a spreadable consistency, fill a plastic piping bag (or freezer bag) with the filling, trim off the tip then pipe the chocolate into the centre of each cupcake. Alternatively, spoon it into the centres using a teaspoon.
Fit a large piping bag with a large star nozzle then fill the prepared bag with the frosting. Pipe swirls of frosting onto each filled cupcake and immediately decorate each one with sprinkles so that the sprinkles stick to the frosting. Alternatively, you can spread the frosting onto each cupcake with a spatula or blunt knife.
Chocolate filling can be made up to 2 days ahead and kept in the fridge until needed. To soften, leave out at room temperature for 2-3 hours, or heat in microwave on lowest power setting for 10 second bursts, mixing in between, until a soft enough consistency.
Cupcakes will keep for 2-3 days at room temperature stored in an airtight container.