Prepare a standard muffin tin with 12 paper liners.
Combine chocolate, peanut butter and salt in a heavy-based saucepan over very low heat, or use a double-boiler, to melt the chocolate. Stir frequently. Remove from heat.
Place 2 teaspoons melted chocolate in each cupcake liner and carefully spread it with the back of a spoon at the bottom and only ¼ of the way up the liner. Repeat with all 12 liners.
Place the chocolate-covered liners onto a plate in the fridge to firm up, for about 20 minutes.
Meanwhile, prepare the filling: Stir together peanut butter, sugar, optional crumbs, and salt.
The chocolate should be hard by now, so divide peanut butter paste into all 12 liners, about 1 heaping tablespoon (20g) per liner, pressing down gently to make sure the paste goes everywhere. Place in fridge to firm up, for about 20 minutes.
Top with 2 teaspoons melted chocolate, spreading carefully so that none of the peanut butter can be seen.
Let cups firm up in fridge for at least one hour before enjoying. Store in fridge or freezer.