Butter and line a traybake or small roasting tin, about 20 x 30cm. Heat oven to 180C/160C fan/gas 4. Gently melt the butter in a large saucepan, cool for 5 mins, add sugar, vanilla and eggs, then beat until smooth with a wooden spoon. Stir in the flour, cocoa and ¼ tsp salt. Stir in the milk chocolate drops and bake for 35 mins until risen all over and a skewer comes out with a few damp crumbs.
For the top, gently heat 85g butter and 85g caster sugar together until both are melted. Stir in 200g light condensed milk and bring to a boil. Cool for 5 mins, then stir in 50g milk chocolate drops to melt. Spread over the cold cake, scatter with more chocolate drops and cut into squares.