Preheat the oven to 180℃ (160℃ fan)/350°F/gas mark 4. Line two baking sheets with nonstick baking parchment or a reusable silicone liner.
Place 150g each milk and dark chocolate in a large heatproof bowl and add 125g butter Place over a saucepan of gently simmering water and stir with a spatula until the chcolate and butter has melted and combimned together. Remove the bowl from the pan and allow to cool for a couple of minutes.
Add 100g granulated sugar and 125g light muscbado sugar to the melted chocolate and stir in. Lightly beat the two eggs and egg yolk together with a fork and add to the chocolate mixture along with 1 teaspoon vanilla extract. Stir well with a wooden spoon to combine. Sieve 225g flour, 40g cocoa powder and 1 teaspoon cornflour into the batter. Mix together well, checking the bottom of the mixture to ensure there are no pockets of flour. Leave the batter for 4 or 5 minutes to allow it to firm up a little.
Use a teaspoon to create balls of dough, of roughly equal size. Place onto the prepared baking sheet, spacing them well apart. Slightly flatten the cookie dough balls with your hand to create thick discs.
Place the baking sheet into the oven and bake for 10 minutes. Allow the cookies to cool for 5 minutes on the baking sheet. Use a pallet knife or fish slice to carefully transfer them to a wire rack to finish cooling and firming up.
To make the peanut butter filling
Place 125g butter into a mixing bowl. Beat to ensure it is nice and soft. Add 125g peanut butter and mix well to combine. Sieve about a third of the 150g icing sugar into the buttery mixture and mix together. Continue adding the icing sugar and mix until it is well combined.
To complete
Once the cookies have completely cooled and firmed up, pair them to those of equal size and shape. Half fill a piping bag fitted with a plain round nozzle with the peanut butter frosting. Turn a chocolate cookie over. Pipe a circle of frosting around the circumference of the cookie, about 3 - 4mm from the edge. Pipe a spiral of frosting within the circle. The underside of the cookie will now have a coating of frosting. Sandwich the cookie with its pair. Repeat until all of the cookies have been sandwiched together.
Enjoy!
Notes
Cook's NotesYou may want to weigh the balls of raw cookie dough to ensure equal size, this will aid matching them up when they are sandwiched together. My balls of dough weighed 26 - 27g.After baking for 10 minutes will be puffed up and will be soft. On cooling the cookies will have flattened and will be starting to firm up.StorageThe cookies will keep for upto a week if stored in an airtight container in a cool place.