Pre heat oven to 200c / 390f fan forced
Place potatoes, evenly spaced, on a large, rimmed baking sheet lined with baking/parchment paper (see note) and bake for 30 mins
To make the homemade ranch, combine mayonnaise & sour cream, mixing until smooth
Add garlic powder, milk & vinegar to mayo, mixing to combine (see note) & set aside
Remove tray from oven, and while holding potatoes steady with tongs, with a sharp knife cut each potato in quarters & open up slightly to cool (about 2 mins)
Meanwhile, in a small microwave safe bowl, melt butter, then add cumin, paprika & ground coriander seeds mixing to combine
Using a melon baller, scoop out the flesh from each potato quarter, and place scooped flesh back on to tray amongst the skins
Using pastry brush, brush the melted butter mixture over the potato (see note) sprinkle with salt & pepper
Bake for 22 - 30 mins, or until crispy & well browned (see note)
Serve immediately with homemade ranch dressing