Preheat the oven to 180°C (350°F) and grease a 9" or 20cm round pie dish. Alternatively you could use an 8" or 9" square baking pan.
Crush the digestive biscuits and mix with the melted butter.
Press into the base and up the sides of the pie dish and place in the oven for around 10 minutes or until it turns a light golden.
Next make the fudge.
Melt the chocolate and pour into a bowl with the condensed milk and vanilla extract and mix until well combined.
Pour in a an even layer over the biscuit base.
Top with the marshmallows and return to the oven for 5-10 minutes. How long depends on how your marshmallows roast as every oven and marshmallow brand is different.
Check after 5 minutes and if your marshmallows have spread slightly and turned golden on top then they are roasted.
Eat straight out of the pie dish with a spoon of place in the fridge to set for two hours before slicing and serving.