Heat oil to 375 F (190 C). You can use a deep fryer, or a heavy pot. If not using a deep fryer, use a deep, heavy pot, filled to at least 4" deep.
In a large pan, melt butter. Add onion, cabbage, and celery, sprinkle with a little salt and saute until tender.
Add ground pork, garlic, sesame oil, ginger, and chilies, stirring occasionally until pork is browned and cooked through, and liquid has all cooked off.
Transfer pork mixture to a food processor, along with green onions, and blitz until very smooth. Season with salt and pepper to taste, set aside.
Lay out an egg roll wrapper on your workspace. Using a pastry brush (or just your fingers!), brush a bit of water around the entire edge of the wrapper – about a 1" border. Measure about ¼ cup of filling in the center of the wrapper.
Fold one side of the wrapper across the filling, to about ⅔ of the way across the wrapper. Fold the other side of the wrapper across the first side, effectively folding the sheet into thirds, while enclosing the filing.
Gently squeezing air out as you work, press firmly down on all edges, to seal. Repeat for remaining filling / wrappers.
Carefully place egg rolls - seam side down - in preheated oil. Fry, turning occasionally, until crispy and golden.
Transfer to plate lined with paper towels, blot any excess grease before carefully transferring to serving plate.
Serve hot, with plum sauce