Whisk the beer yeast and sugar in a large bowl to blend then set it aside in a warm place until the yeast dissolves about 15 minutes.
Gradually whisk in the flour to form a thick batter. Bubbles in the batter should fall slowly from your finger. Cover the batter with a wet tea towel and set aside until larger bubbles form about 45 minutes.
Bring a large pot of water to a boil over high heat. Cut the potatoes into 5-inch-long by 1/2-inch-thick sticks. Cook the potatoes in the boiling water for 5 minutes.
Drain and allow the potatoes to cool completely on a baking sheet lined with paper towels.
Blend the egg yolks mustard and vinegar in a food processor. With the food processor running slowly drizzle in the oil through the feed tube until the mixture has thickened to a mayonnaise-like consistency.
Transfer the mayonnaise to a bowl then stir in the cornichons parsley capers lemon juice and enough water to thin the sauce to the desired consistency. Season the sauce to taste with salt and pepper and refrigerate until serving.
Fill a clean large wide wok or pot about halfway with oil. Heat the oil over medium-high heat until it reaches 375°F on a deep-fry thermometer. Adjust the heat as needed to maintain this temperature.
Season the cod with salt and pepper and dredge the cod in the flour to coat lightly.
Working in batches dip the cod into the batter to coat and then gently lower it into the oil.
Fry until the cod is golden brown about 4 minutes. Using a slotted skimmer remove the cod from the oil and place it onto a baking sheet lined with a rack to drain the excess oil.
Return the oil to 375°F. Working in 3 batches fry the potatoes in the oil until golden and crisp about 5 minutes. Using the slotted skimmer transfer the chips to the baking sheet lined with a rack to drain. Season the fish and chips with salt while they are still hot.
Arrange the fish on plates with the chips and serve with the tartar sauce and lemon wedges.