Remove steaks from refrigerator. Sprinkle with sea salt and press 1/4 teaspoons of pepper into each side of steaks. Set aside.
Pat potatoes until very dry with paper towels. Pour vegetable oil in large pot to a level of 1 1/2", (or heat oil as directed in deep fryer). Heat oil to temperature of 270 degrees. Cook potatoes in small batches until tender, 2-3 minutes. Do not allow to brown. Drain potatoes on paper towels and cool completely at room temperature (can make up to 3 hours in advance).
Melt butter and olive oil in large, seasoned cast iron skillet over med-high heat. Sear steaks in skillet until browned and crusty, 6-7 minutes a side for medium rare. Transfer steak to warmed platter.
Reheat oil in pot to 375 degrees. Ad potatoes and fry until brown and crispy, about 5 minutes. Drain on paper towels and immediately sprinkle with sea salt and pepper.
Reheat cast iron skiller to med-high heat. Add Cognac to skillet and cook until liquid evaporates. Add cream and beef broth, boiling until sauce is reduced to about 1/3 cup, scraping to loosen browned bits in skillet. Season to taste.
Spoon sauce on individual warmed plates, Top each with fillet. Serve fries along side fillets.