Peel the potatoes using a vegetable peeler and remove any blemishes. Cut into large, even chunks so they cook evenly.
Place the potatoes in a large pot and cover with cold, salted water. Bring to a boil, reduce heat, and simmer for 15–20 minutes, or until fork-tender.
While the potatoes cook, add the cottage cheese and sour cream to a food processor. Blend until completely smooth and creamy, scraping down the sides as needed. Set aside.
Drain the cooked potatoes and let them stand uncovered for about 5 minutes to allow excess moisture to evaporate before mashing.
In the same pot, add half of the butter and the chopped garlic. Place over medium heat and cook together until the butter is melted and the garlic is fragrant, about 1 minute. Remove from heat.
Return the drained potatoes to the pot with the melted butter. Mash using a potato masher until mostly smooth. Pour in the warm milk while mashing to reach your desired consistency.
Add the whipped cottage cheese and sour cream mixture to the mashed potatoes. Stir until fully incorporated and creamy.
Season with salt and black pepper to taste. Stir in the Parmesan cheese, if using, until melted and combined.
Transfer to a serving bowl and drizzle with the remaining melted butter. Garnish with parsley and serve warm.