Spinach & smoked salmon egg muffins

INGREDIENTS

• 1 tbsp white wine vinegar
• 4 of the freshest eggs you can get
• 300g spinach
• 25g butter , for frying and spreading
• 2 muffins , split
• 4 long slices good-quality smoked salmon
• 2 egg yolks
• 140g butter , melted
• juice ½ lemon
• pinch cayenne pepper