Gently heat together the toffees, marshmallows, butter and vanilla in a large saucepan, stirring all the time.
Once melted bring to a simmer for 1 minute then remove from the heat. Tip in the Rice Krispies and stir well. You may need to add a few more if the mixture is too sticky.
Line a Swiss roll tin or similar tin plus another tin about half that size with baking parchment and tip in the mixture. You will also need a small piece of parchment to press it down with.
Place in the fridge until completely cold then remove and cut into squares. You could also fill bun cases with the mixture if you prefer. Store in a tin or airtight container. These will keep well for a couple of weeks, but usually only last a couple of days in my house!