For the caramel shards:
Have a sheet of greaseproof paper ready on a heatproof surface. Measure the 2 tablespoons of water into a heavy-bottomed pan. Cover with the sugar and place over a medium-low heat until the sugar starts to dissolve. Resist the temptation to stir the pan as this can cause crystallisation. Once the sugar starts to dissolve, turn up the heat to medium-high until the sugar is a dark golden brown.
Pour the caramel onto the greaseproof paper in a thin and even layer. Leave to cool, then use a sharp knife to chop into smallish shards. Be careful, as the caramel can be quite hard.
Preheat the oven to 160C/140C (fan)/gas 3. Butter a 30 x 20 cm baking tin and line with parchment paper.
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, and add to the egg and butter mixture along with the milk chocolate pieces. Mix until just combined.
Pour the mixture into your prepared baking tin and smooth the top with a palette knife or spatula. Sprinkle 75g of 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. These will keep well for up to 3 days in an airtight container.