Twixt

1. 1In a small bowl beat together egg and vanilla extract until yolk is broken up and evenly combined; set aside.
2. 2Combine flour, sugar, salt, and baking powder in the bowl of a food processor and pulse a few times to aerate and break up any lumps. Add butter and pulse until mixture looks like sand, about 25 (1-second) pulses. Add egg mixture and pulse just until dough comes together, about 10 (1-second) pulses. Form into a flat, rectangular disk, wrap in plastic, and refrigerate at least 1 hour.
3. 3Heat oven to 375°F. Place dough on a 14-inch piece of parchment paper, lightly flour, and roll into a 13-by-8-inch oval, about 1/8 inch thick. (Work quickly, because the dough will become difficult to roll as it warms up.) Transfer parchment paper with dough to a baking sheet, cover in plastic wrap, and refrigerate at least 15 minutes.
4. 4Trim dough to a 12-by-7-inch square and cut into 3-1/2-by-3/4-inch cookies (you need at least 24). Pierce each cookie four or five times with a chopstick or the base of a thermometer.
5. 5Place on a baking sheet and cook until golden brown, about 15 minutes. Remove to a wire rack and let cool. Meanwhile, make the caramel.

For the caramel:

1. 1Spray a 13-by-9-inch baking pan with cooking spray, then line the pan with a 16-by-13-inch piece of parchment paper, leaving a few inches of overhang on each side. Set aside.
2. 2Combine all ingredients except vanilla extract in a heavy-bottomed 4-quart saucepan fitted with a candy thermometer and place over medium-high heat. Stir mixture until sugar completely dissolves, about 2 minutes. Wash down the inside of the pan with a wet pastry brush to prevent crystallization. Boil mixture, swirling pan occasionally (but not stirring), until syrup is at 248°F, about 8 minutes.
3. 3Immediately remove the saucepan from heat, stir in vanilla extract, and pour caramel into prepared baking pan. Using an oiled rubber spatula, spread caramel evenly in the pan. Immediately press 24 cookies, pierced side down, into caramel, leaving space between them to cut them apart later.
4. 4Let cool until caramel is no longer warm to the touch and holds a slight indentation when pressed with your finger, about 40 minutes. Place filling in the refrigerator until caramel is firm and can easily be cut through, about 40 minutes.
5. 5Remove filling from the baking pan to a cutting board, caramel side down, and, using a sharp knife, cut around each cookie. Peel off parchment paper, place undipped candy bars on a cutting board, caramel side down, and trim away excess caramel. Immediately place on a parchment-lined baking sheet, cookie side down, and set in the refrigerator until caramel is hard, at least 10 minutes. (Note: It’s best to work in a cool area for this step.) Meanwhile, temper chocolate.

For the chocolate coating:

1. 1To temper chocolate, fill a large bowl with 2 inches of cold water, add 3 to 4 ice cubes, and set aside.
2. 2Bring a saucepan filled with 1 to 2 inches of water to a simmer over high heat; once simmering, turn off heat. Place 18 ounces of the chocolate in a dry heatproof bowl. Set the bowl over the saucepan and stir until chocolate is completely melted and reaches 118°F. (Make sure chocolate does not come in contact with water or exceed 120°F. If either happens, start over, as the chocolate is no longer usable.)
3. 3Remove the bowl from the saucepan. Add remaining 6 ounces chocolate and stir until all chocolate is melted and cools to 80°F. To speed the cooling process, after all chocolate has melted place the bowl over the reserved cold-water bath. Meanwhile, take undipped candy bars out of the refrigerator.
4. 4Return the bowl to the saucepan and stir until chocolate reaches 86°F; immediately remove from heat. Do not remove the thermometer from the bowl; check the temperature periodically to make sure it stays between 85°F and 87°F. (Chocolate must remain in this temperature range while dipping or it will not set up properly.) Keep the saucepan over low heat and use it to reheat chocolate as necessary.
5. 5To test if chocolate is properly tempered, spread a thin layer on parchment paper and place it in the refrigerator for 3 minutes to set. If chocolate hardens smooth and without streaks, it is properly tempered. (If it is not properly tempered, you need to repeat the process.)
6. 6Line a baking sheet with parchment paper. Drop candy bars one at a time, cookie side down, into tempered chocolate. Cover caramel side with more chocolate, then remove candy bar. To do so, hold two dinner forks in one hand, crossing the ends of the handles to form a V, with the tines pointed outward. As you remove each candy bar, tap the forks several times against the edge of the bowl and scrape the bottom of the forks across the edge to wipe away any excess chocolate.
7. 7Place Twixt on the baking sheet by tilting the forks so the edge of each candy bar touches the parchment-lined pan, then smoothly pull the forks out. Repeat until all candy bars have been dipped. Let sit at room temperature until completely set, at least 20 minutes.
8. 8Trim any excess chocolate from edges of candy bars and place Twixt in an airtight container. Twixt will last up to three weeks in the refrigerator or up to two months in the freezer. Let come to room temperature before serving.