Line a small baking sheet with parchment paper. Melt butter in a large saucepan over medium-high. Add walnuts, ¼ cup honey, and 3 tablespoons cold water. Bring mixture to a boil over medium-high; reduce heat to medium-low, and simmer, stirring often, until glaze thickens to coat walnuts, about 8 minutes. Remove from heat; pour candied walnuts onto baking sheet, and use a spatula to quickly separate nuts. Let walnuts cool completely, about 15 minutes. Wash and dry saucepan.

Pour oil to a depth of 2 inches into clean saucepan; heat over medium-high to 350°F. Stir together mayonnaise, condensed milk, lemon juice, soy sauce, and remaining ¼ cup honey in a large bowl; stir until well incorporated. Set aside.

Place a wire rack inside a large rimmed baking sheet. Stir together flour, baking powder, garlic powder, black pepper, and salt in a separate large bowl; add remaining ⅓ cup cold water. Toss shrimp in batter until fully coated. Remove shrimp from batter, shaking off any excess. Working in batches, add shrimp to oil in saucepan, and fry until golden brown, about 4 minutes per batch. Drain shrimp on a wire rack.

Transfer shrimp and candied walnuts to honey mixture in a large bowl; fold mixture using a rubber spatula until shrimp are evenly coated. Garnish with scallions and sesame seeds.