Preheat oven to 400 degrees. Place the 10 by 12-inch rectangle puff pastry on a lightly floured work surface with the short side in front of you and cut it into four 3-inch wide strips from left to right. Next cut each strip into 6 triangles. Arrange the triangles on the work surface with the tips facing you. Gently pull each tip toward you; this light stretch adds layers to the finished croissant without adding density. Place a small amount of chopped ham on the wide side of each triangle, about 1/2-inch from the edge. Use the tips of your fingers to roll the triangle from the base to the tip. Place the croissants seam side down on a parchment paper-covered baking sheet. Combine egg wash ingredients and brush the tops of the croissants. Bake until well risen and golden brown, about 20 minutes. Cool on a wire rack before serving. All of these puff pastry items are best eaten the day they are made.