Soak the gelatin sheets in cold water for 10-15 minutes
Chop the white chocolate into small pieces and place into a heatproof bowl
Heat the cream, sugar, salt, vanilla and zest just before boiling point and add it to the chocolate. Let it sit for one minute and stir gently to melt the chocolate.
Squeeze the gelatin in your hand, add to the chocolate and stir. Add the milk, stir to combine and refridgerate for 4-6 hours or overnight.
Crush the cookies into fine crumbs (by hands or in a food processor).
Put them in a heatproof bowl with butter and heat on microwave oven on high in 30 seconds increments until very warm and the butter melted. Stir well.
If you have leftovers, they keep well in the fridge, in an airtight container, for up to 3-4 days.
My gelatin sheet packet has 12 sheets and weights total 20 grams. So each sheet is 20/12 = 1.6 grams, and for this recipe I use 2 sheets (about 0.2 grams more which is OK)