Homemade Klondike Bars

Ingredients

• For the filling:
• 3 ounces egg whites
• 3 3/4 ounces sugar
• 1/4 teaspoon Diamond Crystal kosher salt; for table salt, use about half as much by volume or the same weight
• 1/4 teaspoon cream of tartar, or 1/4 ounce lemon juice
• 8 ounces heavy cream
• 2 1/2 ounces milk, any percentage will do
• 1/8 teaspoon vanilla extract
• For the chocolate coating:
• 7 ounces dark milk chocolate, such as Endangered Species 48%, finely chopped
• 3 1/2 ounces refined coconut oil

Directions

1. Start by fitting parchment into an 8-inch-square cake pan: use 2 sheets, each 8- by 14-inch, crossing them so the base and walls are covered and a bit hangs over every edge. You will need that overhang to lift the frozen filling out later, so skip wax paper and tinfoil. For the water bath, set a thick coil of crumpled tinfoil in a wide pot; it will hold the bowl above both the pot bottom and the water. Heat the pot on high until the water is bubbling hot, then lower the heat enough to keep it at a gentle simmer.
2. For the filling, put the egg whites, sugar, salt, and cream of tartar in the stand mixer bowl. Set the bowl over the simmering bath, resting it on the foil coil, and make sure it stays clear of the water. Cook while constantly stirring and scraping with a flexible spatula until the egg white mixture registers 165 degrees F (74 degrees C). In a metal bowl, expect this to take about 6 minutes; if it drags on much longer, the heat is too low. If it sets too quickly or turns scrambled even though you are stirring the whole time, the water is either boiling or reaching the bowl.
3. Once it hits 165 degrees F, move the bowl to the stand mixer, attach the whisk, and beat on high until the meringue turns glossy, stiff, and thick, about 5 minutes. The exact time depends on how much power your mixer has.
4. When the meringue is thick and stiff, beat the heavy cream until it also holds stiff peaks. Use a separate bowl with a hand mixer or a whisk, or move the meringue elsewhere and use the original stand mixer bowl; the whisk does not need washing. Whisk the milk and vanilla into the meringue, then add the whipped cream and whisk until the mixture is smooth.
5. Right away, scrape the "ice cream" into the lined pan and smooth it level. Cover the pan with foil, keeping the foil off the surface, and freeze overnight or until the mixture reaches 0 degrees F (-18 degrees C). At the same time, put a cutting board in the freezer; it should be large enough to hold the pan.
6. Lift the frozen ice cream out of the pan by pulling up on the parchment overhang. Peel off a parchment sheet and lay it over the top of the ice cream, then flip the slab onto the chilled cutting board. Remove the other parchment sheet and, with a large chef's knife, cut the ice cream into 9 squares, cleaning the knife with a paper towel after each cut. Loosely cover the bars with plastic wrap and put them back in the freezer until you are ready for them. If they will sit for more than 1 hour, wrap them snugly in plastic so they do not pick up freezer odors.
7. For the chocolate coating, melt the chocolate over a water bath, or use a microwave-safe bowl and heat it with 2 or 3 15-second bursts on normal power, stirring thoroughly after each one. Stir in the coconut oil until the coating is completely melted and smooth. If stubborn bits remain, warm it briefly again and stir until they disappear. Pour about two-thirds of the coating into a bowl or container about 4 1/2 inches wide and 2 1/2 inches deep, then let it cool to about 80 degrees F (27 degrees C). How long that takes will depend on how warm the coconut oil and melted chocolate were at the start.
8. To assemble, wait until the chocolate has cooled, set out an extra parchment sheet, and take the ice cream bars from the freezer. Work with one bar at a time: slide an offset spatula underneath to release it, then move it into the chocolate coating. Use a fork to submerge the bar, lift it out, and let the extra coating drip away. Set the dipped bar on a clean piece of parchment on the open side of the cutting board, so the finished bars gradually take over the chilled space as you go.
9. Repeat until every bar is coated, replenishing the dipping bowl with the reserved chocolate when you need it. If the bars begin to melt or the edges start to soften, stop and return them to the freezer before continuing.
10. Freeze the coated bars until the chocolate is fully set, about 15 minutes, then move them to a freezer-safe zip-top bag with a gallon capacity. If desired, wrap each bar in wax paper or thin decorative foil first. Freeze again until the ice cream firms back up after being out of the freezer, about 4 hours more. Stored airtight, the bars will keep in the freezer for up to 1 month.