Combine flour and dry yeast in the bowl of a stand mixer with the bread hook.
Add warm water and butter to bowl, mix well. Mix in the sourdough starter.
Add the remaining ingredients and 1 cup whole wheat flour, a little bit more if needed, until you get a slightly sticky ball of dough that sticks together. The quantity of flour will depend on the air humidity.
Knead with the dough for 8 to 10 minutes, by hand or in the mixer. It should be elastic.
Oil the mixing bowl and place the dough in it, turning it once to oil the dough. Cover with a damp cloth and rise for 1 hour, or until doubled in volume.
Punch down gently and cover with a damp cloth and rise for 1 hour, or until doubled in volume.
Divide dough in two and shape as desired or into two logs to be baked in a prepared loaf pan. Leave to rise, covered, for 1 hour or until doubled in volume.
Bake in a preheated oven at 375 F for 45-50 minutes, rotating halfway through, or until the top is a deep golden brow and a thermometer inserted inside reads 210 F. Cool for 1 hour on a wire rack before slicing.