Step 1In a small saucepan over medium-high heat, warm the milk until bubbles begin to form around the edges. Remove the pan from the heat, add 4 tablespoons (half a stick) butter and one-fourth cup syrup and cool until the butter has melted and the mixture is warm to the touch (about 110 degrees). Stir in the yeast and allow to sit until the mixture is bubbly, 5 to 10 minutes.
Step 2Meanwhile, place the flours and salt in the bowl of a standing mixer fitted with a dough hook and combine over low speed; alternatively, whisk together the flours and salt in a large bowl.
Step 3Stir the mustard into the bubbly yeast mixture, then slowly pour the mixture into the flour while mixing over low speed, until the flour is incorporated.
Step 4Mix the dough until it is soft, smooth and elastic, about 5 minutes in a mixer and 10 minutes by hand. Add a little flour as needed if the dough is too moist. Turn the dough out onto a lightly floured board or surface and knead for a minute or so. Place the dough into a large buttered bowl, turning the dough so the surface is lightly coated with butter. Cover the bowl with a towel or loosely with plastic wrap and allow to rise in a warm place until doubled, 1 to 1 1/2 hours.
Step 5Line 2 baking sheets with buttered parchment paper. Punch down the dough and divide it into 24 pieces. Roll each piece into a smooth ball and place the balls on the parchment, spreading them out in even rows on the two sheets. Cover loosely with buttered plastic and allow to rise in a warm place until almost doubled, about 1 hour.
Step 6Preheat the oven to 400 degrees. Melt the remaining 2 tablespoons butter and mix together with the remaining maple syrup to form a glaze. Brush the rolls evenly with the glaze and place the trays in the oven. Bake the rolls until golden brown, about 15 minutes, rotating the trays halfway through for even baking. Remove from the oven and place the rolls, still on the parchment, on racks to cool completely.
Step 7Store the cooled rolls in sealable plastic bags for up to three days at room temperature, or freeze for up to one month. Reheat the rolls (at room temperature) on a tray in a 350-degree oven for 3 to 5 minutes before serving.