Preheat the oven to 350°F (175°C).
Grease a 13 x 9 inch baking pan.
In a bain marie or a saucepan over very low heat, melt the dark chocolate with the butter, stirring constantly. Set aside to cool.
Beat the eggs and sugar together well with a whisk.
Stir in the chocolate and butter mixture with a wooden spoon.
Add the orange flavouring.
Mix the orange rind with the flour.
Add the flour gradually to the chocolate mixture, folding it in gently but thoroughly with the wooden spoon.
Pour into the prepared baking pan.
Sprinkle the milk chocolate chips evenly over top.
Bake for 25 to 30 minutes or until a toothpick inserted near the centre comes out virtually clean (the center should still be a bit squidgy).
Cool completely before cutting into squares to serve.