How to Make It
Place the sugar and 1 cup warm tap water (105° to 115° F) in a large bowl; sprinkle with the yeast and let stand until foamy, 4 to 5 minutes. Whisk in 2 tablespoons of the butter.
Using a wooden spoon, mix in the salt and then the whole-wheat flour. Adding ½ cup at a time, mix in 1½ cups of the all-purpose flour, until dough is soft and just holds together.
Turn the dough out onto a lightly floured surface and knead until smooth and elastic, 5 to 10 minutes, adding up to ½ cup more all-purpose flour as needed to keep the dough from sticking.
Butter a large bowl; add the dough and turn to coat. Cover the bowl with plastic wrap and let stand in a warm place until the dough has doubled in size, about 1 hour.
Butter a 9-by-5-inch loaf pan. Turn the dough out onto a lightly floured surface and fold the dough over itself 3 times. Form into a 9-inch log, pinching to seal the ends; transfer to the prepared pan. Cover the pan loosely with plastic wrap and let stand in a warm place until the dough has doubled in size, about 30 minutes.
Heat oven to 400° F. Remove the plastic wrap from the pan and, using a sharp knife, make several ¾-inch-deep slashes in the top of the loaf. Brush with the remaining tablespoon of butter and bake until the bread is golden and sounds hollow when tapped on the bottom, 30 to 35 minutes. (To test it, turn it out of the pan and tap it with your fingers. A dull thud means that the bread is still moist inside and requires more baking.)
Cool for at least 30 minutes before slicing.