This recipe scales easily. I made about 1/3 the first time just using a part sheet and folding it over. Wasn't too fussy about the quantities. Didn't seem to matter.
Yield 60 pieces
Ingredients
flour, all-purpose, (for surface)
1/4 cup Dijon mustard, plus more for serving
250 grams ham, cooked thinly sliced
150 grams Swiss cheese, thinly sliced
1 large egg
1 teaspoon cream
Preparation
Preheat oven to 400°F
Roll out 1 package of puff pastry on a floured parchment paper to two 16x12" rectangles. Transfer to baking sheets and chill both sheets of pastry until cold, about 15 minutes.
Transfer parchment with pastry to a clean surface. Spread ¼ cup mustard evenly over 1 sheet of pastry, leaving a ½" border around the edges. Layer ham, then cheese over mustard. Fold border of pastry up over edge of filling. Brush edges lightly with water. Arrange remaining sheet of pastry on top, then trim edges with kitchen shears or a sharp knife so top layer fits over bottom with no overhang. Crimp edges with a fork. Using parchment, slide pastry back onto baking sheet. Chill, uncovered, at least 30 minutes.
Lightly beat egg and cream in a small bowl to blend. Brush egg wash evenly over puff pastry. Cut thin slits in a decorative pattern into top of pastry to allow steam to escape. Bake feuilleté, rotating pan halfway through, until puffed and deep golden brown, 30–35 minutes. Let cool slightly before cutting into 1½" squares. Serve warm with additional mustard alongside.
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Do Ahead: Feuilleté can be assembled 12 hours ahead. Cover with plastic wrap and keep chilled.