Preheat the oven to 350 degrees F. Line a 24-cup mini muffin pan with paper liners; coat with cooking spray. Melt the butter and unsweetened chocolate in a heatproof bowl set over a pot of simmering water, stirring, until smooth. Whisk in the granulated sugar, vanilla and salt until combined; remove the bowl from the pan. Stir in the eggs, one at a time. Add the flour and beat with a wooden spoon until the batter is shiny, about 1 minute.
Divide the batter among the prepared muffin cups. Bake until a toothpick inserted into the center comes out clean, about 17 minutes.
Meanwhile, make the filling: Whisk the peanut butter, 1/4 cup confectioners' sugar and 2 tablespoons heavy cream in another heatproof bowl set over the pot of simmering water until smooth, about 2 minutes. Transfer to a pastry bag fitted with a small round tip.
Let the brownies cool 5 minutes in the pan; make an indentation in the center of each with a teaspoon. Pipe the filling into the indentations.
Make the glaze: Combine the semisweet chocolate and 2 tablespoons cream in a heatproof bowl set over the simmering water. Let sit 3 minutes, then stir until smooth. Stir in the remaining 2 tablespoons confectioners' sugar. Slowly stir in the remaining 2 tablespoons cream until pourable. Spoon over the brownies; top with the coarse sugar. Chill until set, 30 minutes.