Preheat the oven to 160℃ (320℉) and line the brownie tin with greaseproof paper (parchement).
Fill a sauce pan a ¼ full with water and place onto a hob on a medium heat. Add butter, chocolate and sea salt into a mixing bowl and place onto the saucepan making sure the bottom of the bowl does not touch the water. Leave to melt, stirring occasionally. Once melted keep to the side too cool.
Into the stand mixer bowl add egg yolk, whole eggs and sugar. Whisk until it thickens, increases in volume and is a pale in colour. When you lift the whisk out of the bowl it should form a ribbon. Start on a low speed to break the eggs down and slowly turn the speed up to high.
Add the melted chocolate and butter in ⅓ increments and gently fold in scraping down the wall and bottom of the mixing bowl, then fold in the oil.
Sift flour and cocoa powder to remove and lumps, add ⅓ at a time and fold through the mixture until completely incorporated. Followed by the chocolate chips.
Spread half of the mixture into the brownie tin and spread evenly, scatter the raspberries over the mixture and cover with the remaining mixture and spread evenly. Into the oven for 23-30 minutes.Leave to cool completely at room temperature before portioning.
