In a large mixing bowl, combine the warm water, sugar, and instant yeast. Add the softened butter, salt, and bread flour. Stir until a shaggy dough forms.
Transfer the dough to a lightly floured surface and knead for 8–12 minutes, until smooth and elastic. The dough should feel soft and slightly tacky.
Place the dough in a lightly greased bowl, cover, and let rise in a warm area for 60–90 minutes, or until doubled in size.
Gently press down the dough to release excess air. Divide evenly into two portions and shape each into a loaf. Place into two greased 8½-by-4½-inch loaf pans.
Cover loosely and let rise for another 30–60 minutes, until the dough is visibly puffy and slightly above the edges of the pans.
Preheat the oven to 350°F. Bake for 25–35 minutes, or until the tops are golden brown and the internal temperature reaches 190–200°F.
Remove from the oven and brush the tops with melted butter, if desired. Cool in the pans for 5–10 minutes, then transfer to a wire rack to cool completely before slicing.