Blend 50g of fresh blackberries and add just enough water to obtain 90ml of total liquid.
Separate egg yolks from whites.
Sift together in a large bowl the flour with baking powder, add brown sugar and salt.
Make a hole in the center and pour in this order (do not mix now): oil, egg yolks, blended blackberries, lemon zest and vanilla extract.
Add a few drops of lemon juice to egg whites and whip until stiff.
Mix the ingredients in the bowl until you have a smooth batter, add finally the egg whites stirring gently with a spatula from top to bottom.
Pour the cake batter into a 7 inch chiffon cake mold, do not oil or flour it. If you use a traditional cake mold, however, you have to grease it with a little oil and flour it well.
Bake at 310°F for 40 minutes; check if the cake is done with a toothpick, it must come out clean.
Let the cake cool in the mold, turned upside down (if using the chiffon cake mold).
Once the cake has cooled down, remove it from the mold. If necessary, pass a flat knife near the edges of the mold to help the cake come out.
Melt gently in a double boiler or microwave the white chocolate and milk.
Top the chiffon cake with this white chocolate sauce and decorate with remaining fresh blackberries.