For the peanut butter layer, beat soft cream cheese with butter until smooth. Beat in icing sugar until pale and fluffy, then mix in vanilla and peanut butter. Mix in melted white chocolate. It will stiffen up. Stir in milk to loosen it to a spreading consistency.
Spread the peanut butter mixture evenly over the cooled brownie base. Refrigerate for about 30 minutes.
For the topping, combine chopped chocolate with coconut oil in a heatproof bowl and melt in the microwave on high power in 20 second bursts until smooth and glossy. Or, you can melt over the stove on medium-low heat. Pour it over the chilled peanut butter layer, sprinkle with sea salt flakes and let set.