In a shallow bowl whisk flour with cayenne, chili powder and salt. In another shallow bowl, mix egg with milk. In another shallow bowl add panko.
Cut Avocados from Mexico in half, remove pits and slice halved avocados into 4 equal slices. With a large spoon, scoop out whole slices from the avocado peel. Set slices aside.
In a large dutch oven, bring canola oil up to 350 degrees according to a deep fry or candy thermometer attached to the side of pan. Prepare a large plate with paper towels for draining taco shells. One at a time, add corn tortillas to oil. Let tortillas heat for 20 seconds then using long handled tongs, form tortillas into the shape of wide bottomed shells. Cook for 30 seconds more, rotating to fry all sides and remove from pan. Place on draining plate to cool. Repeat until all shells are fried.
Prepare another draining plate with paper towel. One by one, dip avocado slices into flour mixture, then egg mixture, then toss gently in panko crumbs. Add avocado slices to hot oil and fry in small batches until golden brown, about 1 to 1 ½ minutes. Remove from oil with a slotted spoon or tongs and drain on prepared plate with paper towels.
Spread 1 to 2 spoonfulls of refried beans to bottom of taco shell, top with slivered cabbage, then stuff shells with 2-3 fried avocado slices. Top with tomatoes, cotija cheese and Serrano cream sauce. Garnish with cilantro leaves and serve immediately.
For the Serrano Cream Sauce
Add serrano pepper, cilantro and green onions to a food processor and pulse until finely chopped. Add remaining ingredients and blend. Add kosher or more lime juice to taste.