Matasuegras Murcianas (Custard-Filled Fried Biscuits)

Instructions

1. For the pastry cream, heat 400 ml of the whole milk with the sugar, cinnamon stick and lemon zest in a saucepan over medium heat until steaming and starting to simmer, about 5 minutes.
2. Whisk the remaining 100 ml whole milk with the cornstarch and egg yolks in a bowl until smooth. Whisk a ladle of the hot milk into the yolk mixture, then pour it all back into the saucepan.
3. Cook over medium-low heat, whisking constantly, until the cream is very thick and bubbles for 1 to 2 minutes, about 4 minutes total; it should reach at least 160°F (71°C). Remove the cinnamon stick, scrape the cream into a shallow dish, press plastic wrap on the surface and chill until cold and firm, about 2 hours.
4. Lay out half the María biscuits. Spoon about 1 heaping tablespoon (30 g) of chilled pastry cream onto the center of each and top with a second biscuit, pressing gently so the cream spreads almost to the edge.
5. Heat the sunflower oil in a deep skillet to 350°F (175°C). Stir the dark rum into the lukewarm whole milk in a shallow bowl, and put the beaten eggs in a second shallow bowl. Mix the sugar and ground cinnamon for finishing on a plate.
6. Working with 3 or 4 sandwiches at a time, dip each in the rum milk for just 3 to 5 seconds (any longer and it falls apart), then coat in the beaten eggs. Fry until golden, 45 to 60 seconds per side, and drain briefly on paper towels.
7. While still warm, roll each matasuegra in the cinnamon sugar. Arrange on a tray and chill at least 1 hour before serving so the biscuits soften around the cream.