Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and grease and line a 12" x 9" tin
Melt the dark chocolate and butter together in a pan on a low heat. When fully melted, stir together and remove from the heat. Let it cool for 5 minutes
In a bowl whisk together the eggs, vanilla extract and caster sugar until frothy
Add the chocolate mixture to the egg mixture, whisking constantly as you pour it in
Add the plain flour, cocoa powder, vanilla extract and baking powder. Whisk in until you can't see anymore flour
Pour the batter into the lined traybake tin and bake for 15 minutes
While it is baking, make the pecan pie topping by mixing together the maple syrup, light brown sugar, eggs, vanilla extract, salt and melted butter
When the mixture is thick and syrupy, add the pecans and stir until they are all coated
Take the brownies out of the oven after the have been baking for 15 minutes and gently spoon the pecan pie topping evenly all over them
Bake for a further 20 minutes, or until the pecan pie topping is golden all over, allow to cool fully before removing from the tin and slicing - I recommend slicing with a serrated knife as it makes it easier to cut through the pecan layer
Store leftovers in an airtight container in a cool place and eat within 3 days