Peel, Cut, and Soak Potatoes: using a vegetable peeler, carefully peel the skin from russet potatoes. Use either a knife and cutting board or a french fry cutter to then slice potatoes into sticks about 1/4 - 1/2 of an inch thick. Place cut potatoes in a large bowl of cold water and allow them to soak for at least 1 hour.
Mix Spices & Sauce: while potatoes are soaking, prepare sauce and spice mixture. In the first small mixing bowl, combine all Cajun Dipping Sauce ingredients. Stir until well combined, cover with plastic wrap, and refrigerate until ready to use. In the second small bowl, combine kosher salt, garlic powder, and pepper. Set aside until ready to use.
First Fry: once potatoes have soaked, use a colander to drain the water. Rinse potatoes and use paper towels to blot dry them as much as possible. Add 1.5 L of frying oil to a large stove pot, and use a candy thermometer to bring oil temperature to 325F. Once oil reaches temperature, use a slotted frying spoon to carefully add the first batch of fries to pot. Do not overcrowd pot, this will need to be done in 2-4 batches. Fry first batch for 5 minutes, just until heated and starting to change color. Use slotted spoon to gently remove first batch and transfer to a paper towel lined baking sheet. Bring oil back to 325F and repeat this process with remaining batches.
Second Fry: bring oil temperature up to 375F. Carefully add 1 batch of pommes frites back to pot and fry again for 60-120 seconds until golden brown. Watch them closely as they can change color quickly. Remove the first batch from the pot and transfer to a clean paper towel to drain grease. Immediately sprinkle fries with a generous amount of salt mixture. Allow oil temperature to come back up to 375F. Repeat this process with remaining batches.
Add Garlic & Parmesan: melt 1 tbsp of butter, and mix in 2-3 cloves of minced garlic. Pour melted butter over fries and toss to coat. Sprinkle grated Parmesan and dried parsley over fries and toss again to fully combine. Serve immediately with Cajun Dipping Sauce.