Heat oven to 200C/180C fan/gas 6 and line a 25 x 18cm baking tin with baking parchment. Put the oats in a bowl with a pinch of salt.
Gently melt the butter in a small pan with the sugar and syrup. Once melted, remove from the heat and allow to cool for a couple of mins. Grate the orange zest into the pan and squeeze in the juice. Pour into the oats and stir thoroughly, until it’s all combined, then add three quarters of the chopped chocolate.
Pour the whole mix into the baking tin and press down with the back of a spoon. Press the remaining chocolate into the tray, then bake for 15-20 mins, until golden. Cut into squares and remove from the tin. Allow to cool completely on a wire rack before serving.