Preheat the oven to 350.
Line cookie sheets with parchment or silpats.
In a high-heat-friendly glass bowl set over a pot of gently boiling water, melt the butter, 1 cup of the chocolate chips and the chopped unsweetened chocolate.
Set the bowl aside to cool, or transfer the melted mixture to a large mixing bowl.
Cool for 5-10 minutes.
Add the sugar, flour and baking powder and stir to combine.
The mixture will look grainy.
Once the batter is no longer hot to the touch, add the eggs and vanilla and combine.
The mixture will begin to shine.
Using a spatula, incorporate your additional chocolate chips and/or the other add-ins.
Using a tablespoon, dollop your coookie dough onto the lined cookie sheets.
If you are using a candy "topping" like a Mars Bar slice or a bit of Andes Mint, place it on the centre of each cookie now.
Bake for 8-10 minutes, until the cookie is shiny but not dark or wet towards the middle. The cookies will still be soft to the touch, though. So, don’t over bake!
Cool cookies on the pan for 3-5 minutes.
Move the parchment or silpat sheets of cookies to wire racks and cool completely.
[Optional re the Mars Bar variation] After cooling, if you really need to have the cookies make a tidy appearance, use kitchen shears to carefully trim off the caramelized bits and drips of candy bar that may be sticking out from the edges of your cookie. You can save those scissored-off bits as ice-cream toppings!