Start by lining your square baking pan with parchment paper and pre-heating your over to 350°F.
Place half of your chocolate chips (6oz) into a heatproof bowl and set this over a saucepan of simmering water. This should fit snuggly. Avoid letting the water touch the base of the bowl, but allow this to carry on simmering on a medium-low heat on the stovetop while you melt the chocolate.
While the chocolate is melting, whisk the oil, vanilla, eggs and yolk together in a large mixing bowl.
Once the chocolate has melted smooth, let this cool for a couple of minutes and then slowly add to the egg mixture while whisking continuously. You want to avoid scrambling the eggs with too hot chocolate!
Next add the granulated and brown sugar to the mixture and sift the flour, powdered sugar, cocoa powder and salt into the mix. Stir this together with a spatula until just combined in a thick batter.
Add the remaining chocolate chips to the batter and fold to incorporate.
Spoon this into your lined baking pan and spread evenly.
Bake in the center of the preheated oven until a toothpick inserted in the center comes out mostly clean.
Remove and allow to cool completely in the pan for about an hour or two.
When the brownies are cool, melt the marshmallows in a small saucepan over a medium-low heat until smooth.
Remove from the heat and allow to cool for a few minutes until you are able to handle the marshmallow - it will be warm, but shouldn't be so hot to the touch that it burns!
Pinch a bit of melted marshmallow with one hand and then pinch this with your other, pulling the marshmallow apart. The stretching action will create a spider web effect. Drape this over your brownies letting it cling to the edges to hold in place. Repeat until you've added as much webbing as you like!
Top off with some edible eyes or other ghoulish candy decorations. Slice and serve!