Preheat oven to 350 degrees Fahrenheit.

Prepare a double boiler.

Dice the dark chocolate.

Melt diced chocolate and butter in the double boiler until smooth.

Remove from heat.

Pour the melted chocolate and butter mixture into a bowl.

Stir in flour and powdered sugar until well combined.

Separate the yolks from the egg whites.

Add the yolks, one at a time, to the chocolate and flour mixture, stirring after each addition.

In a separate bowl, beat the egg whites with a pinch of salt until stiff peaks form.

Gently fold the egg whites into the chocolate mixture, being careful not to deflate them.

Prepare your mold(s).

Pour the chocolate mixture into the mold(s).

Cut the Mars bars into small pieces.

Sprinkle the pieces of Mars bars on top of the mixture in the mold(s).

Bake for 20 minutes.

Let cool for 5 minutes before removing from the mold(s).

Serve immediately with custard or a scoop of ice cream.