Preheat the oven to 200 degrees C.
Put the milk, eggs, caster sugar and vanilla extract (if using) in a bowl or large jug and beat well with an electric whisk, or else by hand. Sift in the flour and beat well again, then leave to rest for 5 minutes.
Put a little butter in 10 'cups' of a standard muffin tin.*
Put the tin in the oven for 5 minutes to melt the butter and heat the tin, just as you do with Yorkshire Puddings.
Take the tin out and quickly pour in the batter to fill each one by about 2/3. I made 10 with this amount. Add chocolate to each one, then quickly put back in the oven and bake for 25 minutes until they are well risen (mine almost doubled in height), and golden. When it puffs up and bends slightly at the top, it is said to have 'popped over', hence the name.
Remove from the tin (mine came out easily, but use a palette knife if needed) and serve immediately, while warm.
Serve on their own or drizzled with chocolate sauce. In America, whipped cream is popular too, but I am not sure if I could manage this and chocolate sauce in the morning! I think creme fraiche could be manageable though, perhaps instead of the sauce. You could serve with nuts, as I did, and / or mini marshmallows, or fresh berries.
Optional chocolate sauce (go on!):
Ten minutes before the end of the cooking time, break the chocolate into pieces and put them in a saucepan with the sugar and water over a medium heat. Stir until the chocolate has melted and the sugar has dissolved and it is starting to bubble. Turn down the heat to low and simmer for about 5-8 minutes, stirring often, until a little thicker and syrupy. Drizzle over the popovers. (Can be made in advance and rewarmed in a saucepan).