Preheat and prepare: Set your oven to 200°C (180°C fan) and line a 12-hole muffin tin with paper cases. The oven should be fully up to temperature before you mix the batter, as the chemical reaction in the raising agents begins immediately upon mixing.

Whisk the dry ingredients: In a large mixing bowl, combine the wholemeal flour, cocoa powder, baking powder, bicarbonate of soda, salt, and golden caster sugar. Whisk them together until the cocoa has dispersed evenly – you will see a uniform, dark brown powder with no pale streaks of flour remaining.

Combine the wet ingredients: In a separate jug or small bowl, whisk the eggs, sunflower oil, buttermilk, and vanilla extract until they are fully emulsified. The mixture should look pale yellow and slightly frothy on top, with a smooth, uniform consistency.

Mix the batter: Pour the wet ingredients into the bowl of dry ingredients. Stir gently with a wooden spoon or silicone spatula, using a folding motion. Stop mixing as soon as you see the last streak of flour disappear – the batter will look lumpy and slightly thick, which is exactly right. Overmixing at this stage will make the muffins tough and dense.

Fold in the peanut butter and chocolate: Add the peanut butter in heaped spoonfuls, then scatter in the dark chocolate chips. Fold the mixture just three or four times, leaving visible ribbons of peanut butter running through the batter. This creates pockets of gooey peanut butter that will melt during baking, giving you that signature surprise in every bite.

Fill the muffin cases: Divide the batter evenly among the 12 paper cases, filling each one almost to the top – about three-quarters full is ideal. The batter will be thick and sticky; use two spoons or a cookie scoop to portion it neatly. You should hear a soft thud as each spoonful drops into the case.

Bake to perfection: Place the tin in the centre of the oven and bake for 18–20 minutes. At the 16-minute mark, check them: the muffins will have domed beautifully, and the tops should spring back when you press them gently with a fingertip. A skewer inserted into the centre should come out with a few moist crumbs clinging to it, not wet batter. The kitchen will smell richly of toasted peanuts and deep chocolate.

Cool and serve: Remove the tin from the oven and let the muffins rest in the tin for 5 minutes. This resting time allows the structure to set, preventing them from collapsing. Then, transfer the muffins to a wire cooling rack. The bottoms will feel slightly soft and the paper cases will peel away easily. Serve warm for the ultimate gooey experience, or allow them to cool completely for a firmer texture.