In a large bowl, or the bowl of a stand mixer, combine the butter and both sugars on a medium/high speed for about 3 minutes. Add in the egg, peanut butter and vanilla and combine again.
Add in the flour, baking powder and baking soda. Mix until all combined to a thick cookie dough. Chill in the fridge for 30 mins (or leave overnight).
Preheat the oven to 200C/180C Fan/Gas 6. Line 3 large baking trays.
Scoop the chilled cookie dough into balls (about 1/2 tbsp each) and place on the baking trays (1.5 inches apart, they spread a little). You should have approx 20 balls. Press down with a fork to form the classic criss-cross pattern.
Bake for 10-12 minutes, until golden and crisp at the edges. Let the cookies cool on a wire rack.
Once cool, add 1 tsp Nutella to half of the cookies on the flat side. Sandwich together with a similar cookie.
Melt the milk chocolate in the microwave in 20 second intervals until fully melted. Dunk half the sandwich cookies into the chocolate before placing onto a wire rack. Optional: sprinkle over the mixed chopped nuts. Let set/place in the fridge to set before eating!