In the bowl of a stand mixer, mix the bread flour, whole wheat flour, and salt until the ingredients are evenly distributed. Using a butter knife or your fingers, break the room temperature butter into small cubes and add to the flour and salt mixture. Using the whisk attachment, mix the ingredients in a stand mixer on low for 2-3 minutes until crumbly. (You can mix by hand until crumbly as well).
Remove the whisk attachment and replace it with the paddle attachment. In a separate bowl and using a fork, whisk the water, starter, and honey together with a fork until frothy. Add the starter mixture to the flour mixture and mix with the paddle attachment on low until the ingredients are evenly combined and a dough has formed. The dough will be slightly sticky to the touch. Cover the dough with a damp towel or plastic wrap and let rest for 30 minutes. This allows the gluten in the dough to rest, making it easier to shape.
BULK RISE: After the dough has rested, knead the dough for 8-10 minutes on a lightly floured surface, or for 6-8 minutes with a KitchenAid or stand mixer (speed 1- 2 with dough hook). Next, transfer the dough into a large bowl. Cover the bowl with plastic wrap (this will allow the dough to retain moisture better than a towel) and let rise at room temperature overnight or until it has doubled in size.
SHAPING THE DOUGH: In the morning, coat an 8.5 x 4 inch bread pan with an even layer of butter. Remove the dough from the bowl and place onto a lightly floured surface. Flatten the dough (in order to remove any air pockets) and shape the dough into a rectangle. Fold the long sides of the dough until they meet at the center. Fold the short sides of the dough upward. TIP: It helps to have the bread pan next to the dough in order to gauge length. Flip the loaf over and place into the buttered bread pan with the seam side facing down.
SECOND RISE: Allow your dough to rise at room temperature until the top is about 1 inch above the bread pan. This can take 1-3 hours. Be careful not to let the dough get too tall. This can cause air pockets in your bread. Your bread will rise more in the heat of the oven.
TIP: If you shape your bread in the morning but need to leave the house and return in the evening (work, errands, etc), you can place your loaf in the fridge to slow the second rise and set it back out at room temperature to finish rising once you are home.
BAKE: Preheat the oven to 375 degrees Fahrenheit. Bake the dough on the center rack for 45-50 minutes or until the top is golden brown. Remove from the oven and take bread out of the pan to cool. Slightly peel back the wrapper of a stick of butter and apply a thin layer to the top and sides of the loaf. Wait for the loaf to cool before slicing. Enjoy by itself or warmed up with butter and honey!