Lay out 30 mini muffin liners (petit-fours papers) on a large plate or baking sheet. Set up a double boiler over medium heat. Melt the chocolate in the double boiler, stirring the chocolate as it melts, and when it is entirely smooth, 7 to 10 minutes, remove it from the heat.
Scoop up a bit of chocolate with a small spoon, pour it into a mini muffin liner, and use the back of the spoon to spread the chocolate around the entire inside of the liner. You want the layer of chocolate to be thin, but not so thin that it won’t hold up when it dries and gets peeled out of the paper. Repeat with the remaining cups. Set them aside to harden a bit, 10 to 15 minutes, and wash the spoon. Set aside the remaining melted chocolate.
Meanwhile, make the filling. Combine the peanut butter, vanilla, salt, and brown sugar in the bowl of a food processor fitted with the chopping blade. Process the mixture until it is smooth and uniform.
Even if the chocolate is not yet hard, with the small spoon, fill each cup three-fourths of the way with the peanut butter mixture.
Return the double boiler to medium heat, and soften the remaining chocolate in the bowl. With a large spoon, cover the peanut butter with melted chocolate to the top of the cups to seal in the filling. When all of the cups are filled, leave them to harden at room temperature for at least 3 hours.
These can be stored at room temperature in a covered container for up to 2 weeks, in the fridge for up to 1 month, and in the freezer for up to 3 months.