For the peanut butter salted caramel: Heat the sugar and 1 1/2 tablespoons water in a medium saucepan over medium heat until the sugar melts and the mixture reaches a deep amber color, about 2 minutes. Avoid stirring the mixture to prevent it from crystallizing.
Remove the pan from the heat and immediately whisk in the cream, milk and salt. The mixture will bubble up violently. Once it has settled, add the peanut butter and whisk to combine. Pour the caramel into a glass jar and allow it to cool for at least 1 hour, by which point it should be thick and cool to the touch. The caramel can also be made the day before.
For the chocolate coating: In a double boiler over medium heat, melt the chocolate and butter together.
Place 10 mini cupcake liners into a mini cupcake tin. Pour roughly 3/4 teaspoon of the melted chocolate into the bottom of each cupcake liner and spread it along the sides with a teaspoon so that it coats the entire liner. Place them in the fridge for 5 minutes.
Place 1/2 teaspoon of the cooled caramel into each chocolate liner. Use your finger to slightly flatten the caramel, then pour the melted chocolate over the top, filling the liners to the top. Decorate the tops with crushed peanuts or sea salt flakes if you wish! Put the peanut butter cups back in the fridge for 10 minutes, then serve immediately.