Add the softened butter, vanilla into a bowl with the sugar, and cream together until pale and fluffy. I used a stand mixer but an electric whisk will work, or by hand with a balloon whisk.
Add the flour and a pinch of salt and mix in until just combined, (if should be like small rubble) now make the cocoa dough the same way.
Remove about 1/3 of the mixture and place in a separate bowl and add 2 tbsp cocoa powder and mix in using your hands. (wash your hands so they are clean so you can work with the lighter dough.
Take the bowl of plain dough, using your hands bring it together into a dough, trying not to overwork it too much.
Take a small hand full of the plain dough and roll it into a thin snake, (pencil like) about 6 inches long. place to the side until you need it (about 2 to 3 tbsp amount of dough should be enough)
Tip the remaining plain dough onto the plastic wrap, press down into a large thin rectangle, slightly winder than the dough you already rolled.
mix the swirl ingreidents together and tip on top of plain down and press town making a layer on top and roll over gently just to stick it into the dough,
Place the piece of dough you rolled into a snake at one end, make sure it is the same width, roll it longer if you need too,using the plastic wrap, roll the rectangle of dough around the length of dough until it just meets and gently cut it, you want to roll until you have completely covered the first length of dough, now you will have a new thin log, place the new log on top and continue this step again. using the plastic wrap and pressing down on the dough each time so it sticks. this way you get rings of dough each time. if you prefer just roll the whole thing up into a spiral. like a pinwheel.
The dough will be delicate and may crumble so work carefully using the plastic wrap to roll each time.
Once all the dough is rolled around, cover tightly with plastic wrap and squeeze it slightly to make sure its pressed together while in plastic wrap to keep its log shape and place in the freezer while you prepare the bark.
Place some plastic wrap on the work top and tip the cocoa dough onto it, add a layer of plastic wrap on top. making sure the width is the same as the dough you have already rolled, gently press the dough into a flat rectangle large enough to cover the dough
Take the rolled dough out of the freezer, gently remove plastic wrap and place on to the cocoa dough, and roll it around it using the plastic wrap. press on.
Tightly wrap in in the plastic wrap and press it slightly to make sure it is all stuck together and place it in the fridge for at least 30 minutes, ideally 1 hour.
Preheat the oven to 180c 350f
Cut the dough into slices. place the slices onto the plastic wrap, using the palm of your hand flatten it slightly, pressing to make sure it is all stuck together, (when you cut it there may be gaps, but once its pressed flat it should all stick together and you should see the rings)
There may be a few cracks when you press the dough flat, but this is good as it looks more log like, use a knife to add some more cracks, and add some texture to the bark part of the dough.
Press in some sliced almonds around the edge so they look like mushrooms
Place on a lined baking tray, spaced at least 2 inches apart and bake for 20 minutes
As soon as they come out of the oven, use a knife add more intents to the bark and leave them to completely cool on the tray. once cool remove from tray and enjoy. keep in an air tight container for 3 days