Preheat oven to 350°F. Place pistachios in a small bowl. Unwrap dough and slice crosswise into 18 pieces and roll each between the palms of your hands into smooth balls. Press gently into pistachios to coat half of each ball, then place, pistachio side up, on a parchment-lined baking sheet, spacing about 2″ apart.
Bake cookies until barely golden, 8–10 minutes. Remove from oven and press the handle of a wooden spoon about three-quarters of the way down into the center of each cookie to make a round indentation, and wiggle in a circular motion to widen. Return to oven; bake until golden brown, 5–8 minutes. Let cool on baking sheet.
Using an electric mixer on medium speed, beat pistachio paste and butter in a small bowl until light and fluffy, about 30 seconds. Dollop 1–2 tsp. pistachio mixture into the indentation in each cookie.
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.
Heat oven to 160C/140C fan/gas 3 and line a 12-hole muffin tin with cases. Put 85g pistachios into a food processor with about half the sugar, then whizz until very finely chopped. Tip into a large mixing bowl with the remaining sugar, butter, eggs, flour and milk and beat until smooth. Divide between cases, then bake on a low shelf for 22-25 mins until a skewer poked in comes out clean. Cool on a wire rack.
For the icing, mix just enough water into the icing sugar to get a very thick but still runny icing – just add 1 tbsp of water at a time. Stir in enough food colouring to give a pretty, pale green colour. If any cakes poke above the top of the cases, trim with a small knife, or scoop out with a teaspoon – being very careful not to release any of the case from the sides of the cake. Put a generous spoonful of icing on each cake and let it gently spread to cover. Chop the remaining pistachios and scatter these over with a pinch of edible glitter. Set, then serve.