Grease a large baking tray with a little vegan butter.
Place a very large saucepan over low heat.
Add the vegan butter, vegan marshmallows and vanilla extract. Stir constantly until the marshmallows are melting and stringy.
113 g vegan butter, 566 g vegan marshmallows, 1 teaspoon vanilla extract
Add the Rice Krispies all at once and stir quickly to combine before the marshmallows cool.
230 g Rice Krispies
Transfer the mixture to the greased baking tray and use clean hands or a spatula to press flat into a smooth layer. Leave to cool at room temperature for at least 1 hour.
If desired, place the dark chocolate into a microwave safe bowl and melt in the microwave on 50% power in short bursts of 20-30 seconds until melted. Drizzle over the Rice Krispie treats and leave to set.
100 g dark chocolate
Slice the tray of treats into 12 and serve. Store any leftovers in an airtight container. Do not freeze.