Preheat oven or air fryer to 400 degrees F. Spread fries in a single layer on a rimmed baking sheet or air fryer basket and cook according to package instructions until golden and crispy. Do not overlap the fries or they will steam instead of crisping up.

While fries cook, melt butter in a 2-quart saucepan over medium heat until just bubbling. Whisk in flour and cook for about 2 minutes, stirring constantly, until the mixture turns slightly golden and smells faintly nutty.

Gradually whisk in milk about 1/2 cup at a time, making sure each addition is fully combined before adding more. Stir in salt, pepper, ranch seasoning powder, and 1/4 tsp cayenne.

Reduce heat to medium-low and cook for about 5 minutes, stirring frequently, until the sauce thickens enough to coat the back of a spoon.

Using a silicone spoon, fold in the freshly shredded white cheddar gradually, stirring gently until fully melted and smooth, about 2 to 3 minutes. Keep heat low to prevent the sauce from breaking.

Transfer hot crispy fries to a serving platter. Pour the Cajun ranch cheese sauce generously over the top and finish with the remaining 1/4 tsp cayenne. Serve immediately.