Preheat the oven to 160 C, 140 C Fan, Gas 3. Butter a 30 cm x20 cm baking tin and line with baking parchment.
Gently melt the 250g butter in a small heavy bottomed pan and set aside to cool slightly.
In a large bowl whisk together the eggs, sugar and vanilla until frothy, then whisk in the melted butter.
In a separate bowl whisk together the flour, baking powder and salt, then add to the egg and butter mixture along with the milk chocolate pieces. Whisk until just combined.
Pour the mixture into the prepared baking tin and smooth the top with a palette knife or spatula. Sprinkle the caramel shards over the top and bake for 30 minutes. The centre should be puffed and set but still a little gooey.
Leave to cool completely in the tin, then cut into 12 thick but smallish pieces. They will keep in an airtight container for up to three days.