Cream the Fats and Sugars: In a large bowl, using an electric mixer or a stand mixer with the paddle attachment, beat the softened butter, 150g of smooth peanut butter, caster sugar, and light brown soft sugar together on a medium speed for about 2-3 minutes, until the mixture is light, pale, and fluffy.

Add the Wet Ingredients: Add the egg and vanilla extract to the bowl and beat again until fully incorporated. Scrape down the sides of the bowl with a spatula to ensure everything is well mixed.

Combine the Dry Ingredients: In a separate bowl, whisk together the plain flour, bicarbonate of soda, and salt. Add this to the wet mixture and mix on a low speed until just combined. Be careful not to overmix here; stop as soon as you can't see any dry flour.

Chill the Dough: Cover the bowl with cling film and place it in the refrigerator to chill for at least 30 minutes. This step is crucial for preventing the cookies from spreading too thin during baking.

Shape and Bake: Preheat your oven to 180°C (160°C fan) and line two large baking sheets with baking parchment. Roll the chilled dough into small balls, about 25g each (a tablespoon measure works well). Place them on the prepared sheets, leaving plenty of space between each one. Using the back of a fork, gently press down on each ball, then press again at a 90-degree angle to create the classic criss-cross pattern.

Bake the Cookies: Bake for 12-14 minutes, or until the edges are golden brown and the centres look just set. They might seem a little soft in the middle, but they will firm up as they cool. I find that rotating the trays halfway through the baking time ensures they cook evenly.

Cool Completely: Let the cookies cool on the baking sheets for 5 minutes before carefully transferring them to a wire rack to cool completely. This can take up to an hour.

Make the Filling: While the cookies are cooling, prepare the filling. In a medium bowl, beat together the 100g of smooth peanut butter, the sifted icing sugar, and the 50g of very soft butter until smooth and creamy. If the mixture is too stiff, add a tablespoon of milk at a time until it reaches a spreadable consistency.

Assemble the Cookie Sandwiches: Once the cookies are completely cool, find matching pairs. Spread a generous teaspoon of the peanut butter filling onto the flat side of one cookie, then gently press another cookie on top to create your sandwich. Repeat with the remaining cookies.