Heat oven to 350ยบ - butter a 9X9" baking pan (or 3 qt casserole).
Peel 4 LG or 6 med. russet (brown baking) potatoes and slice very thinly (5-6 cups total) - set aside.
In a saucepan, melt 4 TBLSP butter over low heat. Stir in 4 TBLSP flour until incorporated. Slowly, add 1-2/3 C milk (whole or 2%), whisk well. Add 1/2 tsp. salt, 1/4 tsp. pepper, 1/8 tsp. garlic powder, 1/4 tsp. seasoned salt (or ms. dash - I use Trader Joe's 21-season salute), a few shakes of cayenne pepper. Continue cooking & whisking sauce over low heat until it begins to boil and thicken. Remove from heat and add 1 cup shredded cheddar cheese and 1/2 cup grated co-jack cheese. Stir well until the cheese is melted & the sauce is smooth.
Layer half the potatoes in the prepared pan. Spoon half of the cheese sauce over the potatoes. Repeat with the remaining potatoes and spread the rest of the cheese sauce over the top. Sprinkle an additional 1/2 cup shredded cheddar (or co-jack or combination of the 2 cheeses) over the top.
Sprinkle paprika on the top and bake, uncovered for 65-75 minutes (poke the potatoes to make sure they're done - if not, bake an additional 10-15 minutes).
