To make the biscuits begin by placing the nuts on a parchment lined baking tray and roast them in an oven pre-heated to 160C Fan/180C.
Roast them for 15 minutes and then when they are golden brown, remove them from the oven, leave them to cool and then coarsely chop them.
Put the butter and peanut butter in the bowl of stand mixer fitted with the paddle attachment and cream the together until they are smooth and creamy.
Add the sugar and beat until creamy, then add the eggs and vanilla and beat until just combined, you may need to scrape the sides down to ensure all of the dough is mixed in.
In another bowl mix the plain flour, baking powder and bicarbonate of soda together and then sift this into biscuit mixture.
Mix slowly to incorporate the dry ingredients without them flying out of the bowl. When this is just mixed with no plain flour showing, add the oats and the nuts and pulse gently until just combined.
Mound the dough onto a sheet of cling-film and flatten it down. Wrap the dough completely and place in the fridge for 30 mins to firm up.
Line two baking trays with greaseproof paper and take the biscuit dough from the fridge, break off pieces of dough and roll them into a shape about the size of a walnut (still in its shell) or slightly smaller than a golf ball.
Place them on the baking tray, about 2 inches apart and flatten the top of the dough so that it is no longer a ball.
Place the trays in the freezer for 15 minutes to chill down before baking. You will need to bake the biscuits in two batches as you will have too much for both baking trays.
Take the remaining biscuits and roll them into balls and place them in the fridge until the other biscuits are cooked.
Preheat the oven to 160C Fan/180C and when it's up to temperature, cook two trays at a time for 8-10 minutes and leave the biscuits on the tray for 5 minutes before transferring them to a cooling rack.
Repeat with the remaining biscuit dough, you should be able to make almost 48 individual biscuits, making 24 biscuits in total.
To make the buttercream place the egg whites in the bowl of a stand mixer fitted with the whisk attachment.
Place 75g of the granulated sugar in a small saucepan with the water. Stir and bring to a simmer over a moderate heat until the syrup reaches 100C.
Start to whisk the egg whites until they become foamy, add the remaining 15g sugar and continue to whisk.
When the syrup reaches 120C, slowly pour it into the egg whites and continue to whisk until stiff peaks and the side of the bowl feels cool (approx 15 minutes).
When the meringue feels cool, reduce the speed and slowly add the butter in small cubes. keep whisking until all of the butter has been added. If the buttercream looks like it's splitting at all, just whisk it quicker.
Then add the peanut butter and whisk it all in together. Place the buttercream into a piping bag fitted with a French tip and place it in the fridge to firm up a little - don't leave it in there for too long as you don't want it to set hard.
When the biscuits are cooled, pair them up with similar sized biscuits, pipe a spiral on one side of the biscuit and sandwich it together with the other biscuit. Place them in the fridge to firm up for at least 30 minutes before eating them.
These biscuits will keep for 3-4 days in the fridge, before they soften too much.