To make the sponge: In a small bowl or large measuring cup, combine the water, yeast, sugar, egg yolks, cream, and milk; set aside., To make the dough: Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. In a large mixing bowl, a stand mixer fitted with a paddle, or the bowl of a food processor, combine the flour, sugar, and salt. Slice the butter into 1/4" pieces and add to the dry ingredients. Work in the butter, mixing with the paddle or pulsing in the food processor until the the butter is in pea-sized chunks., Pour the sponge into the bowl with the flour/butter and blend just until evenly moistened. The dough will be soft but not sticky. Remove from the bowl, pat the dough into a rectangle, wrap in plastic, and refrigerate for 12 to 24 hours., To make the filling: Whisk the eggs, sugar, and flour together in a small saucepan. Gradually add the milk. whisking to prevent lumps. Bring the mixture to a simmer over medium heat, stirring constantly, until it thickens and bubbles all the way to the center. Remove from the heat and stir in the vanilla. Cover with plastic, pressing it onto the surface to keep a skin from forming, and chill thoroughly., To fold the dough: Remove the chilled rectangle of dough from the refrigerator and let it rest on the counter for 10 minutes. Unwrap, place on a floured surface, and roll the dough into an 18" x 10" rectangle. Fold the dough in thirds, like a business letter, and repeat the process one more time. Wrap and rest the dough in the refrigerator for 1 hour before cutting and shaping., To assemble: Divide the dough in half. Roll each half into a 15" square. Cut nine 5" squares from each. Place 1 tablespoon of the filling in the center of each square, and tuck the corners of each square into the custard in the center, pressing down firmly. Transfer the filled squares to a parchment-lined baking sheet. Cover and let rise for 35 minutes while you preheat the oven to 350°F., To make the topping: Beat the egg whites with the milk. Brush on the assembled pastries. Sprinkle the egg-washed pastries with pearl or sparkling sugar., Bake the pastries for 22 to 25 minutes, until golden brown. Remove them from the oven and cool on a rack; when cool, place a spoonful of jam on the center of each pastry.