Preheat the oven to 200C/180C Fan/Gas 4 and line a baking tray with baking paper.
In a large bowl, beat the butter, egg yolk, sugar, honey and salt together until smooth.
Gradually sift in the flour, cocoa powder and baking powder and mix until smooth, but be careful not to overwork the mixture. Add the white chocolate and combine.
Roll the dough into a cylinder roughly 3cm/1¼in in diameter. Wrap in baking paper and refrigerate for 45 minutes, or until firm.
Cut the roll into 5mm/¼in slices and place on the baking tray, leaving a gap between the cookies.
Bake for 15 minutes; they should still be a little soft when you remove them from the oven. Leave to cool slightly before serving.