Place the mayonnaise and custard in a bowl and mix well to combine.
Spread one side of each slice of bread with butter and the other side with the maonnaise mixture.
Divide the cheese, pickle and pastrami between 4 slices of bread, ensuring the buttered side is on the outside, and sandwich with the remaining bread, once again ensuring buttered side is on the outside.
Heat a large non stick frying pan over low heat.
Cook the sandwiches, in batches for 3-4 minutes each side or until golden and cheese is melted.