In a medium saucepan, melt the chocolate and butter together over a low heat while stirring to prevent the chocolate from burning. Stir in the vanilla and salt.
125 g dark chocolate, 110 g unsalted butter, 1/4 tsp salt, 1 tsp vanilla extract
Let the mixture cool for a few minutes while you preheat your oven to 200°C fan (400°F fan) and line an 8 or 9-inch (20 or 23cm) square pan with baking paper.
To the chocolate mixture, add the sugar and stir together well. Crack in the eggs one at a time, beating in quickly after each addition until really well mixed. Stir in the flour.
200 g granulated sugar, 2 medium eggs, 65 g plain white flour
Pour 2/3 of the batter into the tin, and spread so it coats the bottom of the pan. Arrange 12 oreos evenly over the surface of the batter, pushing them down a little, to slightly submerge them. Cover the oreos with the remaining batter. Try to smooth the whole thing out a little by spreading the batter around or pushing the oreos down a little more if they start to float around. Bake for 20 minutes.
16 oreo cookies
This step is optional but I really really really advise it!!: As soon as the brownies have baked, take the pan out of the oven and shove into the freezer on top of a tea-towel (and make sure the edges of the hot pan aren't touching anything plastic). This helps cool the brownies down very quickly so it results in a very gooey middle with a crunchy exterior.
Once the brownies have cooled completely, cut int 12 or 16 squares.
Crush up the remaining oreos (I put them into a sandwich bag and bashed them with a rolling pin in) and sprinkle over the individual brownies. Melt together the chocolate and peppermint extract and drizzle over the brownies*, letting it drip down the edges a bit to secure the oreo crumble to the brownie. Let cool and set.
75 g dark chocolate, 1/4 tsp peppermint oil, 16 oreo cookies
Notes
*I like to let the chocolate cool a bit, pour it into a small plastic sandwich bag then cut a very small amount of one corner of the bag off. It's perfect for drizzling it a neatly as possible over the brownies.