- 1 cup pretzel crumbs (about 2 cups of small pretzel sticks)
- 5 tablespoons unsalted butter, melted
- 2 tablespoons light brown sugar
- 1 (14-ounce) can sweetened condensed milk
- 1/2 cup lime juice
- 2 teaspoons finely grated lime zest
- 1 cup sugar
- 1/4 cup water
- 2 tablespoons light corn syrup
- 4 large egg whites, at room temperature
- 1/4 teaspoon salt
Line a 9- by 5-inch loaf pan with plastic wrap, allowing it to hang generously over the edges. Combine pretzel crumbs, butter, and light brown sugar in medium bowl. Press mixture into bottom and 2 inches up sides of prepared loaf pan. Place pan in freezer.
Whisk sweetened condensed milk, lime juice, and lime zest in medium bowl until smooth; set aside.
Stir sugar, water, and corn syrup in heavy-bottomed medium saucepan over medium-low heat until sugar dissolves. Increase heat to medium-high and boil syrup until it registers 240°F on candy thermometer, 8 to 10 minutes.
While syrup boils, with electric mixer fitted with whisk attachment, beat whites and salt on medium speed until loosened, about 1 minute. Increase speed to medium-high and whisk until soft peaks form, 2 to 3 minutes.
When ready, with mixer running on medium-high speed, slowly and carefully add syrup to egg whites. Beat until outside of bowl is cool to the touch and whites are thick and glossy, about 7 minutes.
Mix about 1/3 of meringue to lime mixture. With rubber spatula, fold in remaining meringue. Scrape mixture into frozen crust and cover loosely with plastic wrap. Freeze at least 8 hours.
When ready to serve, wipe sides of loaf pan with a towel dampened with hot water. Using the plastic wrap, pull the semifreddo up and out onto a platter; discard plastic wrap. Slice and serve immediately.