In a deep fryer, heat oil to 375° F. Combine ½ cup flour, cornstarch, Cajun seasoning, baking powder and salt. Whisk in beer until smooth.
Place remaining flour in a shallow bowl. Season fillets with salt and pepper; then lightly dust with flour.
Dip fillets in beer batter, allowing excess to drip off. Place catfish in oil a few pieces at a time. Fry until well browned on all sides, about 3 minutes.
Remove, and drain on paper towels.
Serve with fresh lemon wedges, malt vinegar and Homemade Tartar Sauce for dipping.
Combine all ingredients in a small bowl; stir to combine. Refrigerate any excess.