Preheat your oven to 160℃ (140℃ for fan ovens) and line a muffin tin with 12 muffin cases.
Mix the milk and apple cider vinegar together in a small bowl or mug and leave it to sit for 5 minutes. It will curdle and become thicker.
Add the milk mixture to a large bowl along with the eggs and egg yolk, the melted butter, the sugar and the vanilla extract. Whisk together until smooth.
Add all of the flour, cocoa powder and salt. Mix well until you have a smooth muffin batter then stir in the chocolate chips.
Divide the batter between your 12 muffin cases and bake for 20-25 minutes, until a toothpick through the middle comes out clean. Then transfer them to a wire rack to cool.
Topping:
Melt the marshmallows in the microwave for around 20-30 seconds until gooey.
Use a teaspoon to stretch the melted marshmallow over the muffins to look like spider webs. This can get really messy so take your time! If the marshmallow starts to set, pop it back in the microwave for 10-20 seconds until it becomes smooth again.
Top each muffin with a decorative or edible spider and enjoy!
Video
Notes
Store in an airtight container in the fridge for up to 4 days.