Preheat the oven to 400°F.
In a small, heavy-bottomed saucepan, over medium-low heat, melt the chocolate and butter, whisking occasionally to combine until smooth. (You can also melt the chocolate and butter in a bowl in the microwave, in two blasts of 30 seconds each, stirring the mixture after each round.)
In a large bowl, whisk the eggs, egg yolks, sugar, and salt vigorously until pale in color. Whisk in the chocolate-butter mixture, followed by the flour, and mix until smooth.
Scoop approximately 2 tablespoons of the chocolate batter into each of the 8 prepared cups in the muffin pan. Next, spoon 1 tablespoon of peanut butter onto the center of each, followed by another tablespoon of the chocolate batter. Use the back of a spoon or your (clean) fingers to smooth the chocolate over the top to cover the peanut butter completely. Press extra chocolate chunks into each if you like it extra chocolaty.
Bake for 14 to 16 minutes, until puffed. Use a small offset spatula and a spoon to gently scoop out each mini cake onto a serving platter or individual plates, and dust with the cocoa and cinnamon powder to serve.