Line a 20 x 30cm tin with baking parchment. Melt the chocolate, butter and golden syrup in a bowl set over a pan of simmering water, stirring occasionally, until smooth and glossy. Add the vanilla, biscuits, sultanas and Rice Krispies, and mix well until everything is coated.
Tip the mixture into the tin, then flatten it down with the back of a spoon. Press in some mini eggs, if using, and put in the fridge until set. When hard, drizzle all over with the melted white chocolate and set again before cutting into chunks.