Preheat the oven to 180°C.
Grease and line an approx. 22x22cm baking tin with parchment paper.
Heat the Guinness in a saucepan on the stove and reduce it by half. Let it cool.
Meanwhile put the chocolate into a thick glass bowl then place on the top of simmering water. When it’s melted add the butter or spread and stir until melted. Let it cool.
Sieve the flour and salt into a bowl.
Combine sugar, eggs, orange zest and vanilla extract in a large bowl. Add the orange juice and Guinness. Stir in the melted chocolate mixture. Gently fold in the flour.
Tip the batter into the baking tin. Bake for 30-35 minutes. It’s ready when a skewer/toothpick inserted into the centre comes out clean.
Cool completely, then cut it into squares. Dust with icing sugar if you like.