1. Grease and line a brownie tin 22x29cm, 8 ½” X 11” with baking parchment. Heat your oven to 180°C.
2. To make the base, cream the butter and sugar together until light and fluffy. Add the egg yolk, vanilla extract and stir. Add the flour & chocolate chips and combine until you have a dough. Knead gently to bring it all together.
3. Press the dough into the base of the prepared tin.
4. To make the brownie mix, break 225g of chocolate into small chunks and place in a heatproof bowl with the butter. Place over a pan of simmering water and heat until melted. Leave to cool.
5. Beat the eggs and sugar together until pale and the mixture has thickened. Add the melted chocolate and butter mixture to the eggs and stir until thoroughly combined. Fold in the flour, salt, chocolate chunks and vanilla extract.
6. Pour on top of the cookie dough and bake for 15 minutes.
7. Scatter the mini marshmallows and crushed pretzels over the surface of the brownie and bake for a further 25 minutes. Keep checking as all ovens are different. A little mixture should still stick to a skewer when inserted in the middle of the brownie.