Make the blueberry swirl first so it can cool while you make the brownie batter. Add the blueberries and Golden Syrup to a small saucepan and simmer to a syrup (takes about 5minutes) and set aside to cool.
Preheat the oven to 180c. I use a square, silicone cake tray so it doesn't need greasing but I would suggest that for a metal cake tin you line with greaseproof paper. It also makes it easier to get out of the tin before slicing!
In a large saucepan, melt the dark chocolate, margarine, sugar, Golden Syrup and milk over a low heat. Make sure you stir it to prevent the chocolate burning or you'll end up with a very stodgy mess!
Remove from the heat and stir in the vegetable oil. Set this chocolate melty mix aside to cool while you weigh out the 'dry' ingredients into your large mixing bowl or stand mixer.
With the stand mixer on minimum, drizzle in the chocolate mixture and stir on low, scraping down the sides and bottom, until completely combined!
Scoop the brownie mix into the prepared tin. Spoon dollops of the blueberry mixture around and then swirl around with a skewer or chopstick.
Bake at 180C for 25minutes until the top has a lovely crust but the inside is still really squidy. If they are for later, leave to set in the fridge before slicing, or eat them like Daddy Peacock - scooped warm on top of a big bowl of ice-cream!
*I've added golden syrup as an ingredient but to be honest I prefer honey or maple syrup - the flavour is much better. Obviously if you are vegan then don't use honey.