1. Generously butter a large bowl; set aside. Place 1 cup milk in a small saucepan, over medium heat. Heat until milk reaches 110 degrees on a candy thermometer. Pour milk into the bowl of an electric mixer fitted with the dough hook attachment. With machine on low speed, add granulated sugar, yeast, salt, butter, lemon zest, orange zest, and eggs. Add flour, and mix on low speed until a soft, slightly sticky dough forms around dough hook. Continue kneading, scraping down hook and sides of bowl as necessary, until smooth, about 4 minutes more.

2. Add currants; knead, with dough hook, to incorporate. Turn dough out onto a floured surface. Knead briefly to evenly distribute currants in dough. Shape into a ball. Place dough in prepared bowl. Turn to coat with butter. Cover with plastic wrap and let rise in a warm place until doubled in size, about 1 hour and 20 minutes.

3. Line 2 baking sheets with parchment paper; set aside. Turn dough out onto work surface. Knead briefly and roll dough into a log. Cut log in half and cut each half into 12 equal pieces. Shape each piece into a tightly formed ball. Place on prepared baking sheets 2 inches apart. Cover baking sheets with plastic wrap and let rise in a warm place until buns are touching and doubled in size, 1 1/2 to 2 hours.

4. Preheat oven to 375 degrees with racks positioned in the upper and lower thirds of the oven. Place bun crossing paste in a pastry bag fitted with a small plain tip or a paper cornet with a 1/8-inch opening. Pipe crosses over the surface of each bun. Transfer buns to oven and bake until golden brown, 18 to 25 minutes, rotating baking sheets after 10 minutes. Transfer to a wire rack and cool to room temperature.

5. Meanwhile, place apricot jam in a small saucepan over medium heat. Cook until heated through; strain through a mesh sieve set over a bowl. Brush heated jam over buns.