Place the 1/2 cup butter in the bowl of a stand mixer or large bowl. Pour hot milk over butter. Add sugar, mashed potatoes, and salt. Allow to cool slightly while you do the next step.
Dissolve yeast in the 1/2 cup warm water. Add to milk mixture with eggs and 4 cups of the flour. Beat until smooth and well blended.
Add remaining flour 1/2 cup at a time until a soft dough forms. When dough comes together, transfer to a clean, well-floured surface and knead for 1-2 minutes by hand. Place in a lightly greased large bowl and cover with plastic wrap. Allow to rise for 45 minutes.
Preheat oven to 350 degrees and lightly grease a baking sheet or large casserole dish.
Roll dough into a large rectangle about 1/2 inch thick. For the filling, spread the softened butter onto the dough. Mix together sugar, brown sugar, and cinnamon and sprinkle over buttered dough. Tightly roll one long side to the other long side and pinch the seam to seal. Use a pizza cutter or sharp knife to cut into about 16 sections.
Place sections swirl-side-up/down onto your greased casserole dish or baking sheet. (see note) Bake for 20-30 minutes until golden and cooked through.
Prepare the frosting. Cream together cream cheese and butter until smooth. Add milk, powdered sugar, and vanilla and mix again for 2-3 minutes until smooth. Drizzle or spread over warm cinnamon rolls and serve.