Preheat the oven to 350F degrees. If using a refrigerated pre-made pie dough, dethaw the dough completely if frozen. Lay the pie dough into a 9 inch pie plate (if not already). Then place the pie plate & prepared crust back in the fridge as you make the brownie.
Put the butter and dark chocolate in a large, heat proof bowl. Microwave for 30 seconds, then stir the mixture with a large wooden spoon or rubber spatula. Repeat the process until the butter and dark chocolate are melted together and smooth. Let the mixture cool for 5 minutes. Alternatively, melt together the butter and chopped chocolate in a double boiler. Add the butter and chocolate to a heatproof bowl. Place the bowl on top of a saucepan with about ½ to 1 inch of simmering water over low-medium heat. Ensure that the bottom of the bowl does not touch the water. Gently stir s the mixture melts together.
Whisk in the sugar, vanilla and salt using a wire whisk. Then whisk in the 2 eggs and additional egg yolk. Make sure the mixture isn't hot when you add in the eggs, or else the eggs can scramble. Then gently stir in the flour using your rubber spatula or wooden spoon.
Remove the prepared pie crust from the fridge and pour/spoon the brownie batter into your prepared crust. Cover the edges of your pie with a pie shield or strips of aluminium foil to prevent burning.
Bake in your preheated oven for 30-35 minutes. After about 15-20 minutes remove the pie shield/aluminium foil so the edges of the pie can brown. When the brownie is done baking, the top should be set and no longer shiny. The brownie will be very gooey and fudgy, but if you prefer it less gooey you'll want to bake it for 35-40 minutes. The one pictured here baked for 32 minutes.
All the brownie to cool fully before digging in. You can cover the pie with foil and store in the refrigerator for up to 4 days.