Preheat the oven to 340°F (170°C).
Lightly grease a square baking pan (about 9 inches).
Mix together milk and vinegar, then set aside to thicken.
Mix in the remaining wet ingredients.
Sift in the dry ingredients.
Gently mix until just combined.
Pour the forest fruits into the bottom of the pan.
Pour the cake mixture on top.
Sprinkle the sugar and cocoa powder over the cake batter.
Pour over the boiling water.
Bake for 30-35 minutes, or until the center is almost set.
Leave to cool slightly before serving!