Preheat the oven to 180°C (350°F) and line a 12-cup muffin tin with muffin liners.

Melt the 80 g butter and let it cool slightly so it does not cook the egg when combined.

Whisk the 150 g caster sugar with the melted butter in a large mixing bowl.

Whisk in the 100 g plain yogurt, 60 g egg, and 1 tsp vanilla extract until combined.

Place the 180 g all-purpose flour, 38 g cocoa powder, 3/4 tsp baking soda, and 1/2 tsp salt in a second bowl.

Fold the dry ingredient mixture into the wet mixture with a silicone spatula until a thick batter forms.

Stir the 120 ml buttermilk into the batter just until it is smooth and no dry streaks remain.

Divide the batter evenly among the 12 muffin liners, filling each liner about three-quarters full.

Sprinkle approximately 60 g chocolate chips evenly over the batter-filled liners.

Bake the muffins at 180°C (350°F) for 18 to 20 minutes.

Check doneness by inserting a toothpick into a muffin; remove the tin when the toothpick comes out clean or with a few moist crumbs.

Cool the muffins in the tin for 5 minutes, then transfer them to a cooling rack until slightly warm.

Warm the 120 g Biscoff spread briefly until pipeable, then transfer it to a piping bag fitted with a round piping tip.

Insert the piping tip into the center of each warm muffin and pipe about 10 g Biscoff spread into each one.