Start by preheating your oven according to the instructions on your fries package. Line two separate baking sheets with foil and give them a light spray of cooking spray. Using two pans instead of one keeps the fries spread out so they actually crisp up in the oven.
In a small bowl, mix together the brown sugar, garlic powder, onion powder, chili powder, paprika, salt, and pepper. Spread the frozen fries across both baking sheets, then sprinkle the seasoning blend evenly over the top of each pan. Save about half a teaspoon of seasoning to sprinkle on later if you like extra flavor.
Toss the fries gently so the seasoning coats every side, then spread them back out into a single even layer. Bake according to the package directions, rotating the pans halfway through if your oven tends to cook unevenly. This step is what gives the fries their crispy, well-seasoned crust.
While the fries are baking, melt the butter in a medium skillet over medium-low heat. Once it's melted, whisk in the flour and let it cook for a minute or two. This quick step removes the raw flour taste and helps the sauce thicken properly later.
Slowly whisk in the milk, making sure to break up any lumps as you go. Let the mixture simmer gently for a few minutes until it starts to thicken up nicely. This forms the creamy base your cheese will melt into.
Add the freshly shredded cheddar cheese a small handful at a time, whisking until each addition fully melts before adding more. Going slowly here is the secret to a smooth, glossy sauce instead of a clumpy one. Once all the cheese is melted, lower the heat to keep it warm without letting it scorch.
Once your fries are done baking, plate about a quarter of them and spoon roughly a quarter cup of the warm cheese sauce over the top. Drizzle with a tablespoon of ranch dressing and sprinkle with any reserved seasoning. Repeat with the remaining fries for however many plates you're serving.
These fries taste best the moment they're assembled, so don't let them sit around too long. Serve with extra cheese sauce and ranch dressing on the side for dipping. The contrast of hot, crispy fries against the cool ranch is what makes this dish so addictive.