Preheat the oven to 180 degrees Celsius (350 Fahrenheit, 160 degrees fan).
Grease the bottom and the sides of an ovenproof dish - mine is 24 cm large and 5 cm high (9 inches large and 2 inches high).
Scatter the light brown sugar over.
Rinse the wild blackberries with cold water, drain well, and transfer them to a large plate lined with kitchen paper to absorb the excess water.
Scatter the blackberries over the brown sugar.
In a food processor, sift the flour, add the eggs, soft butter, vanilla extract, caster sugar, bicarb of soda, baking powder and milk.
Blitz to get a smooth batter.
Spread the batter evenly over the blackberries in an even thin later.
Bake for 25 minutes or until it's the sponge is golden and a skewer inserted in the middle comes out clean.
Remove the cake from the oven, leave it to cool for 10 minutes, then run a sharp knife around the edges to loosen up the sponge.
Place a large plate over the tin, and turn it upside down so that they blackberries are not at the top and the sponge at the bottom.
Leave the cake to cool down completely before you slice it.