Preheat the oven to 170°C/ Fan 150°C/ Gas 3. Grease a 23-cm square tin with butter and line with baking paper, allowing the baking paper to extend out of the sides to make lifting out easier later.
Place the butter, salt, oil and both sugars in a stand mixer fitted with a balloon whisk attachment (or use a handheld electric whisk) and whisk until just combined. Add the eggs and vanilla and whisk on high speed until pale in colour. Sift in the flour and cocoa powder and fold in gently. Add the roughly chopped chocolate and fold in again. Don’t overmix after adding the flour!
Pour the batter into the prepared tin and bake for 20–30 minutes until no longer liquid in the middle, but it should still look technically ‘undercooked’. It will finish cooking as it cools. Leave to cool in the tin for 5 minutes, then lift out using the baking paper tabs, and place on a wire rack to finish cooling. Once cool, wrap in plastic wrap and freeze for 30 minutes(this makes cutting out the shapes a lot easier and neater).
Use circular cutters to stamp out shapes. You can cut out different sizes for different-sized spiders!
Melt the extra 50g chocolate in a microwaveable bowl in 15–30-second bursts. Break up pretzel sticks and attach to the brownies using the melted chocolate. Use more chocolate for where the pretzels join each other. Ensure your chocolate isn’t too warm, as you will be holding the leg in place for a long time before it sticks! Use more melted chocolate to attach the edible eyes.