Cook frozen French fries according to package directions at 425°F until golden and crispy, or prepare fresh potatoes by soaking cut fries in cold water for 30 minutes, patting completely dry, and frying at 350°F until golden.
In a small bowl, combine brown sugar, garlic powder, onion powder, chili powder, paprika, salt, pepper, and cayenne. Mix thoroughly to create the Cajun seasoning blend. Set aside.
Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for 1-2 minutes until the mixture turns golden and bubbly, creating a roux.
Gradually whisk in warm milk, about 2 tablespoons at a time, stirring constantly to prevent lumps. Continue cooking and stirring for 3-4 minutes until the sauce thickens enough to coat the back of a spoon.
Remove the saucepan from heat and stir in shredded cheddar cheese until completely melted and smooth. Taste and adjust seasoning with salt or cayenne if needed. Keep warm over low heat, stirring occasionally.
Transfer hot fries to a large mixing bowl. Sprinkle the Cajun seasoning blend over them and toss until evenly coated.
Arrange seasoned fries on a serving platter. Pour warm cheese sauce evenly over the top. Drizzle with ranch dressing and garnish with fresh chives and jalapeños if using. Serve immediately while hot and crispy.