In a measuring cup, add water and milk. Stir together. Heat it in the microwave for 30 seconds or until the temperature reaches 100˚F-110˚F (38˚C-43˚C).
To a small bowl, add yeast. Pour in ¼ cup (2 oz) of the warm water/milk mixture. Stir and set the mixture aside 10 minutes to activate (becomes frothy).
In a large bowl, combine the flour, sugar, and salt. Mix until combined.
Melt the butter and add it to the remaining warm water/milk mixture and stir.
Make a well in the center of the flour mixture. Pour the water/milk/butter mixture into the well of the flour mixture. Pour in the sourdough starter discard and the yeast mixture.
Stir until the mixture becomes shaggy dough. Turn the dough out onto a floured work surface (or in a stand mixer with the paddle attachment). Knead for 10 minutes adding flour or water, 1 tbsp at a time, until the dough is no longer sticky but tacky.
Return the dough to the large bowl and cover with a towel or plastic wrap. Allow the dough to rise in a warm area for 1-2 hours or until doubled in size.
Grease or line with parchment paper a loaf pan (1.5 quart/liter like 9x5).
Deflate and turn out the dough onto a floured work surface. Knead a few times and place the dough in the loaf pan. Use your hands to roughly shape it to the loaf pan.
Cover the loaf pan with a towel or greased plastic wrap and set aside in a warm area for 1 hour or so until risen.
Preheat the oven to 350℉ (177℃) during the last 30 minutes of the rise. Uncover the bread and place it in the oven to bake 40-50 minutes or until the internal temperature reaches 190°F (88˚C). The bread should be golden brown all around.TIP: If the top begins to brown too quickly, add a piece of tented foil to the top during the last 15-20 minutes of baking. Remove the hot bread from the oven. Place the pan on a wire rack to cool 10 minutes. If using parchment paper, lift the bread out of the pan and place it on a cutting board to cool completely. If using a greased pan, run a knife along the inside pan edges to release the bread and invert it to slide out on a cutting board. Allow the bread to cool completely (1-2 hours) before cutting it.