EASTER CHOCOLATE FUDGE TRAYBAKE

•175ml sunflower oil, plus extra for brushing •225g plain flour •75g cocoa powder •300g caster sugar •2 tsp baking powder •1 tsp bicarbonate of soda •½ tsp fine salt, plus a pinch •2 large eggs •175ml whole milk, plus 2 tbsp warmed •125g unsalted butter, softened •125g icing sugar •20g cocoa powder •2 x 80g packs Cadbury Mini Eggs, roughly chopped