Savoury ham and cheese buns that taste just like Tarte flambee, perfect for a nice brunch or dinner. Just as good the next day!
Warm the milk and ½ cup water in a medium saucepan over low heat until it is about 100°F (but no more than 110°). Remove the saucepan from the heat, and sprinkle the yeast over the liquid. Sprinkle a pinch of the granulated sugar over the top, and set aside without stirring until foamy, about 5 minutes. Then whisk in the butter and egg yolk.
Whisk together the flour, remaining granulated sugar and salt in a large bowl. Make a well in the center, and stir in the yeast mixture with a wooden spoon until you’ve made a thick, slightly sticky dough. Turn the dough onto a floured work surface, and knead until soft and elastic, about 6 minutes. Shape the dough into a ball.
Brush the inside of a large bowl with butter. Put the dough ball in the buttered bowl, turning to coat lightly. Cover the bowl with plastic wrap, trace a circle the size of the dough ball on the plastic and note the time. Let the dough rise in a warm place until doubled in size, about 1 hour 30 minutes.
Turn the dough out of the bowl, and knead briefly to release excess air; re-form the dough into a ball, and return it to the bowl. Lightly butter a large piece of plastic wrap, and lay it on the dough. Cover the entire bowl tightly with the rest of the plastic, and refrigerate for 4 hours or overnight to let the dough proof.
Melt the butter in a medium skillet over medium heat. Add the leeks, and cook, stirring frequently, until softened and just beginning to caramelize about 5 minutes. Sprinkle the leeks with the flour, and cook, stirring constantly, until lightly browned and toasted for about 1 minute. Whisk in the wine and ¼ cup water, and let the mixture thicken, whisking constantly, about 1 minute. Remove the skillet from the heat, and stir in the Gruyere, ½ teaspoon salt and a few grinds of pepper. Transfer the filling to a medium bowl, and let cool completely. (The filling will firm up a bit, but if it’s too warm, it will be difficult to form the rolls.)
To fill and form the rolls: Butter a 9-by-13-inch baking pan with 2 tablespoons of softened butter. Turn the dough onto a floured work surface, and press it flat. Roll it into a 10-by-18-inch rectangle, with 1 long edge facing you. Spread the cooled filling evenly over the dough, leaving about a 1-inch border on the side opposite you. Sprinkle the ham over the filling. Starting from the side facing you, roll the dough up into a tight roll. Lightly brush the 1-inch border with water, and continue rolling to seal the roll. Fold the ends in just a bit, and if any spots look thicker than others, even out the roll with your hands.
Cut the roll every 1 ½ inches with a serrated knife, to make 12 buns. Place the buns cut-side-down (except for the end pieces) in the prepared pan, spacing them 1 inch apart. Cover the buns loosely with plastic wrap. Set in a warm place to rise until the buns double in size, with no gaps in between, 1 hour 30 minutes to 2 hours.
Position the rack in the center of the oven, and preheat to 350°F. Bake the buns until they’re golden brown and the tops spring back when pressed lightly, 25 to 30 minutes. Let cool in the pan for 10 minutes.