Preheat the oven to 350ºF (180ºC).
Butter or spray a 9-inch (20x20cm) square baking pan and line it with a strip of parchment paper. See Notes below.
In a large bowl, melt butter and chocolate, see Notes below for methods.
Add the sugar and mix for 30 seconds.
Add the vanilla and other extracts if using, and mix well.
Add the eggs and beat by hand until very well mixed.
Add the dry ingredients, sifted (flour, cocoa, and salt). Fold well until the batter is shiny and smooth.
Pour the chocolate mixture into the prepared pan and bake for 20-25 minutes, until slightly jiggly in the middle.
Let cool for about 30 minutes on a wire rack.
Run a smooth-bladed knife along the edges and lift the parchment paper to transfer the brownies to a rack to cool completely before cutting.
You can pop them into the fridge for a few hours to make cutting them easier.
They are better the next day, in my opinion.
They keep for a few days and can be frozen for up to a month, well wrapped.