Warm the Greek yogurt in the microwave for about 30 seconds, just enough to take the chill off.

In a large bowl whisk together the white whole wheat flour, bread flour, baking powder, and salt.

In a separate bowl whisk together the starter, warmed yogurt, whole milk, honey, and egg until smooth, then mix in the softened butter until fully incorporated.

Pour the wet mixture into the dry ingredients and mix until a soft slightly tacky dough forms.

Cover the bowl and let the dough rest for 30 minutes.

After the rest, knead the dough in a stand mixer on speed 2 for 5 to 6 minutes or by hand for 8 to 10 minutes until it smooths out and becomes elastic.

Place the dough in a lightly oiled bowl, cover tightly, and let it bulk ferment at room temperature.

Perform one set of stretch and folds at the 30 to 45 minute mark, then leave it alone.

Turn the dough out onto a lightly floured surface and shape it using the rectangle fold method.

Cover the shaped loaf and let it proof at room temperature for 2 to 3 hours until the dough crowns about 1 inch above the rim of the pan.

Preheat the oven to 375 degrees F.

Whisk together the egg and splash of whole milk for the egg wash and brush the top of the loaf gently.

Bake time is about 38-42 minutes.

Remove from the oven and brush immediately with melted butter.

Let the loaf cool in the pan for 10 minutes, then transfer to a wire rack.

Cool for at least 2 hours before slicing.