Easy Cheese and Prosciutto Croissants

1. Preheat the oven to 400 degrees F. Grease a 12-cup muffin tin. 2. Unfold each sheet of pastry. Place the butter slices in an even layer on one sheet of pastry. Place the second sheet of pastry over the butter. Both sheets of pastry should be stacked, one on top of the other. Now roll the pastry out into roughly a 12x14 inch rectangle. Cut into 9 squares. 3. Fit each square inside the prepared muffin tin. Transfer the pan to the freezer to chill. 4. In a medium bowl, whisk together the cream, 1 egg, mustard, thyme, and a pinch each of salt and pepper. Stir in the cheeses. 5. Remove the muffin pan from the freezer and line a piece of prosciutto on the inside edge. Evenly divide the cheese mixture between the pastries. Place the muffin pan on a baking sheet. Brush the top edges of the pastries with the remaining beaten egg, and sprinkle with coarse sugar. Transfer to the oven and bake for 25-30 minutes, until the pastry is deeply golden and the cheese has melted. Serve warm with fresh thyme.