Preheat the oven to 425°F / 220°C. Measure 2 tablespoons of butter. Put half in a 9in / 23cm cast-iron skillet and warm in the oven until melted.
Whisk 40g of all-purpose flour, ⅛ heaping teaspoon of salt, and 1 teaspoon of sugar. Separately whisk ¼ cup of milk, ½ teaspoon of vanilla extract, and 2 large eggs.
Whisk the wet ingredients into the dry until mostly smooth. Melt the remaining butter and stir it in.
Pour into the hot skillet and bake for 15-20 minutes, until puffed and deeply golden.
Sprinkle with 1 tablespoon of lemon juice and a little icing sugar. Add fresh berries and serve immediately.
