Use paper towels to blot the chicken dry, then combine it with all the marinade ingredients in a zip-top bag or bowl. The seasoning should contact the chicken directly rather than mix with excess surface moisture.
Work the marinade around the chicken until every piece is covered, then marinate for 30 minutes to an hour in the refrigerator. Look for an even distribution of ginger and soy seasoning across the pieces.
Bring vegetable oil to 160°C/320°F in a deep pot or pan, using an oil depth of about 3–4 cm/1¼–1½ inches. This lower temperature is for the first fry, not the final browning.
Pour away the unused marinade, blot the chicken on paper towels, and transfer it to a bowl. The surface should remain seasoned but no longer drip with liquid.
Scatter the cornstarch over the chicken and turn the pieces until coated on all sides. A light, powdery covering is the goal rather than a paste.
Lower individual pieces into the oil in small batches and cook for about 2.5–3 minutes, gently shifting any that catch on the bottom. Leave space between pieces so they can fry separately.
Lift each batch onto paper towels and leave it undisturbed for at least 3–4 minutes, continuing with the remaining chicken. The coating won't yet have its final deep golden color.
Remove loose starch crumbs from the oil, then bring it to 190–200°C/374–392°F. Clear oil helps prevent burnt crumbs from clinging to the chicken.
Give the rested pieces a second fry in batches for about 30 seconds to 1 minute. Remove them when the shell looks golden brown and feels crunchy.
Bring the chicken to the table immediately, adding shredded lettuce and parsley if using. The coating has its best crunch while freshly fried.