Preheat oven to 325°F (170°C). Line 2 rimmed baking sheets with parchment paper.

In a large bowl, stir together condensed milk, vanilla, and orange zest; add flaked coconuts, and stir until well combined.

In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites and kosher salt at medium-high speed until stiff peaks form, 3 to 4 minutes. Gently fold egg white mixture into coconut mixture until well combined. Using a 1½-tablespoon spring-loaded scoop, scoop mixture, and place at least 1 inch apart on prepared pans.

Bake until golden brown, about 15 minutes. Let cool completely on pans on wire racks.

Place a large wire rack over a large parchment-lined rimmed baking sheet; spray with cooking spray.

In a small microwave-safe bowl, heat chocolate and oil on high in 10-second intervals, stirring between each, until chocolate is melted and mixture is smooth. Dip bottoms of macaroons in chocolate mixture, letting excess drip off. Place on prepared rack. Alternately, spoon chocolate mixture into a small pastry bag or resealable plastic bag; cut a ¼-inch opening in tip or corner. Place cooled macaroons on prepared rack; drizzle with chocolate mixture as desired. Refrigerate until chocolate is set, about 20 minutes. Using a large offset spatula, loosen dipped macaroons from wire rack, if necessary. Just before serving, garnish with sea salt, if desired.