Boil eggs for 6 mins, place in an ice bath for 15 mins, then peel. Set aside.
On a meat-safe cutting board, cover chicken with plastic wrap, flatten using a meat mallet or heavy bottom pan (~½ inch thickness). Remove plastic wrap and season with kosher salt.
In a shallow dish, whisk water, egg, and cornstarch. Place panko in a separate shallow dish. Prepare a sheet pan lined with a rack for the chicken after frying.
Fill a large cast iron skillet or heavy bottomed pot with canola oil and heat to 350°F. Dip chicken into egg mixture until coated, then dip into panko and coat both sides. Fry in oil ~6 mins per side until golden brown. Transfer to rack to drain.
Boil noodles in water for 3 mins until al dente. Drain well and set aside.
Meanwhile, combine stock, soy sauce, and ponzu sauce in a medium size pot over medium-high heat. Bring to a boil and reduce to a simmer for 5 mins. Reduce heat and add miso paste. Season with kosher salt. Stir in to combine and simmer on low. Add cooked noodles.
Place noodles in serving bowls and ladle broth over top. Slice chicken katsu and place on top of noodles.
Slice eggs in half and place on top. Top with sesame seeds. Serve immediately.
