Using your paddle attachment on a stand mixer, mix butter and peanut butter until homogenous, around 1 minute on a low speed.

Add the sugar and blend on medium speed until well-combined, around 1 minute.

Add the egg yolk and mix. Make sure you are scraping the sides of the bowl as needed and getting underneath the mixture.

Add the flour, malt powder and baking soda, and mix until dough begins to form (it may be crumbly, that's okay).

Chop up the chocolate and peanuts into quite small chunks and add to the mixture.  Mix very briefly, just enough to start mixing it in.

Lay out a length of plastic wrap (cling film) on the counter and transfer dough to it.  Squish together with your hands and make sure there aren't any dry bits.  Bring it together into a rough log shape about 1.5 - 2 inches thick. 

Wrap the plastic around it tightly (including on both ends), then roll the log until it's smooth and round.  Make sure there aren't any air pockets.

Put the log into the freezer for minimum of an hour, or in the fridge for at least 4 hours.  The dough needs to be *properly* cold or the cookies will overbake.  If it's stone-cold frozen, you'll probably need to let it sit at room temp for 15-20 minutes to be able to cut it.

Pre-heat oven to about 360 F (or 340 F convection), or 170 C.  

Using a serrated knife, slice your dough into about 1/2 inch-thick cookies.  Don't panic if it cracks, just squash back together. I think these are nicer if you go on the thicker side. 

Sprinkle a little bit of flaky sea salt on top, if desired (I DO). Bake for around 12 minutes, until the surface is cracked and the edges are golden.

Leave to cool until solid before eating, for the best texture and flavor...they are super delicate, and they'll completely fall apart if you don't let them cool.  These will stay good in an airtight container for at least a week.