Beat the flour, baking powder, icing sugar and butter together in a stand mixer until a soft dough forms. Take ½ the dough out for later. Add the cocoa to the remaining dough and carefully mix it in. Transfer this to a sheet of baking paper and carefully press and roll it out to a thick rectangle, then put another sheet of paper on top and roll it out to 3mm thickness.
Clean the mixer bowl and add the remaining dough, then add the pistachios and beat them in. Roll out the mixture as before, making it as close to the same size as possible. Slide the two dough rectangles onto a baking sheet (you can put them on top of each other) and chill for 30 minutes.
Put the chocolate dough sheet on the work surface and peel off the top piece of paper. Peel the top sheet off the pistachio dough and invert it onto the chocolate layer. Trim the edges to make a neat rectangle, then roll it up along the long edge to make a log shape with a spiral pattern running through it – use the baking paper to help you, as you would when rolling a swiss roll. If the dough is too stiff, let it warm up a bit.
Tip the sprinkles onto a plate and carefully roll the dough in it to coat the outside, then wrap it up in clingfilm and freeze for at least 3 hours.
Heat the oven to 180C/fan 160C/gas 4. Line a baking sheet with baking paper. Cut the dough log into ½cm slices (let it warm up a little if you need to) and space them well apart on the baking sheet. Bake for 15 minutes or until they just start to turn golden. Cool for 10 minutes before carefully transferring to a wire rack to cool.