Add potatoes to a medium-sized saucepan. Add enough water to cover the potatoes, then place on the stove. Bring the potatoes to a boil and continue boiling until they are cooked through. Once cooked, drain the potatoes and reserve 1 cup of potato water. Mash the potatoes, then let them cool for about 20-30 minutes.

In the bowl of a stand mixer, fitted with a whisk attachment, combine warm milk, sugar, and yeast. Lightly whisk together until just incorporated, then let the mixture sit for 5 minutes or until bubbly.

Swap the attachment to a dough hook, then add in all-purpose flour, bread flour, and salt. Knead on medium-low speed until incorporated. The mixture will be clumpy and dry.

Add in potato water and continue to knead until the dough starts to come together, but it will still be a little dry. Add in potatoes and butter and knead until the dough comes together.

Once the dough comes together, knead for 10 minutes on medium speed. The dough will be ready when you can stretch and hold it without breaking it. The dough should also be smooth. Roll the dough into a ball, then place into a large greased bowl. Cover with plastic wrap or a clean kitchen towel, then let rise for 1 hour or until doubled in size.

Once the dough has risen, punch it down, then turn it out onto a clean floured surface. Cut in half, then fold the sides of each half inwards and pinch to hold. Next, fold the edges of each half inwards and pinch to hold. They should look like smooth logs. Place each log into well-greased loaf pans. Lightly cover with plastic wrap and let rise for 1 hour.

Towards the end of the rise time, preheat the oven to 350°F.

Once the loaves have risen, place them into the preheated oven on the middle rack. Bake for about 35-40 minutes or until golden on top and baked through.

Once baked, remove from the oven and place into a wire cooling rack. Let the loaves cool completely, then remove them from the pans and slice.