In a large oven-proof bowl, combine all ingredients except the flour.
Place the bowl in the oven for 15-20 minutes or until the butter starts bubbling and starts turning brown around the edges.
Remove from the oven and carefully add in the flour (watch out for butter fireworks!). Stir until it starts pulling away from the sides of the bowl and sticks together in a ball.
Transfer your tart dough to a 9-inch tart pan with removable bottom and allow to cool slightly or until it is cool enough for you to handle (it's pretty hot!).
Once it cools, start to evenly spread the dough on the bottom and up the sides of your pan (see notes).
Prick the bottom and sides of your dough with a fork.
Bake for 12-15 minutes or until it turns a light golden brown (see notes).
Remove from oven and allow the crust to cool while you work on your filling. Lower the oven temperature to 350F.
For the Chocolate Brownie Filling:
In a large bowl, whisk all ingredients until smooth and thoroughly incorporated.
Pour the batter into your prepared crust and bake for 45-60 minutes or until the filling starts to set around edges but still a little wobbly in the centre (see notes).
Transfer to a rack to cool completely before removing from the pan.
Serve with whipped cream and a dusting of cocoa powder.
The original recipe suggests reserving a small amount of dough for patching up any cracks after the tart shell is cooked. I've found that I never need to do this. If making the crust for the first time, you can do it for good measure.
If you think the sides of your crust are getting too brown too fast, you can loosely cover it with aluminum foil halfway as you wait for the middle portion to catch up.
Do not over bake the tart; otherwise it will get very tough. Start checking for doneness at 45 minutes.