Pour the oil into a large heavy-bottomed stock pot. Heat the oil to 350 degrees F. (Use a thermometer to test the temperature.)
Pour the flour, cornstarch, salt, and baking powder in a medium bowl. Then whisk in the beer, until smooth. Allow the batter to rest for 10 minutes.
Once the oil is hot and the batter has rested, dip the shishito peppers into the batter, one at a time. Shake off the excess batter and slowly lower the peppers into the oil. Fry 6-8 at a time for approximately 3-5 minutes per batch. Once golden, remove with long tongs and place on a paper towel lined plate. Repeat with the remaining peppers and batter.
For the Korean Mayo: Mix the mayonnaise and go chu jang sauce in a bowl. Serve as a dipping sauce for the tempura shishito peppers.