Mix the flour, baking powder and caster sugar in a bowl. Make a well in the middle and add the egg and vanilla extract, if using. Pour in a little of the milk, then beat the egg lightly until it is mixed with the milk and vanilla. Gradually stir in the flour mixture and pour in the remaining milk a little at a time. Before all the milk is added and while the mixture is quite thick, beat well to remove all the lumps, then gradually beat in the remaining milk to make a smooth batter.
Preheat the grill to the hottest setting. On the hob, melt the butter in a 20-22cm frying pan that can safely be used under the grill (cover a wooden handle with foil). Swirl the butter around the pan, then pour in the batter and cook over a medium heat until the pancake is golden underneath. The batter should have risen and almost set on top. The surface will break into little bubbles.
Slide the frying pan under the grill and cook for 1 minute. Scatter the strawberries evenly over the pancake. Sprinkle with the cinnamon and sugar, then grill for 3 minutes or until the sugar has melted. Cut into four wedges and serve.