• Preheat oven to 425°F (220°C). Lightly grease a 6-hole muffin tin (or 6 small ramekins) with butter or cooking oil spray; wipe excess over with a paper towel and set aside.

Stovetop:

• Combine the butter and chocolate in a medium-sized saucepan over low heat. Cook while stirring until melted and smooth.

  ALTERNATIVELY: Combine butter and chocolate in a medium-sized microwave safe bowl; melt in 30 second increments stirring between each until chocolate and butter are melted and well combined.
• Remove from heat (or microwave); stir in sugar and vanilla.
• Add flour to the chocolate mixture in the pan; whisk until combined. Whisk one egg into the mixture at a time until a smooth chocolate batter is formed..
• Pour the batter into the prepared muffin tray (or ramekins) until full. Add 1 marshmallow into each mould and press them down (they will float up slightly but sink while baking).
• Place muffin tray onto a baking tray and bake for 10 minutes or until the edges have set but the centre of each cake is a little soft. Allow to cool in the muffin tray for 5 minutes. 
• Using a knife, gently pull cake away from the sides of the tray before turning out onto a baking tray lined with parchment/baking paper. Lift each cake onto a serving plate with a wide spatula while still warm and gooey (or they will stick to the tray if left for too long).
• To serve, dust with powdered/icing sugar.