Preheat the oven to 200C/180C Fan/Gas 6.
To prepare the berries, mix the frozen berries with the sugar and vanilla in a bowl. Leave to macerate while you prepare the batter.
To make the batter, whisk the eggs and sugar in a bowl until doubled in volume, then fold in the flour. Bring the milk, cream, and almond essence to the boil in a pan, cool slightly, then pour the liquid into the eggs and sugar mixture, while stirring to combine.
Grease a heavy baking dish with butter and dust lightly with flour. Add half of the berries to the dish, pour in the batter, then add the rest of the fruit. Scatter the almonds over the top, then bake for 20-25 minutes.
To make the white chocolate sauce, bring the cream to the boil in a small saucepan, then remove it from the heat and stir in the white chocolate chips.
Serve the clafoutis with white chocolate sauce poured over the top.