1-1/2 cups all-purpose flour
2 teaspoons baking powder
1 teaspoon kosher salt
3/4 cup (1-1/2 sticks) margarine
2 cups brown sugar
1 tablespoon vanilla extract
4 cups roasted unsalted peanuts
2 cups sugar
1/4 cup honey
1/4 cup (1/2 stick) margarine
1/2 cup pareve whippingcream
1-1/2 cups 1-1/2″-wide thin twisted pretzels, coarsely crushed
Preheat oven to 350 degrees. Line baking pan with parchment paper, leaving a 1-inch overhang on long sides of pan. Whisk flour, baking powder, and salt in a medium bowl; set aside. Stir margarine in a medium skillet over medium heat until browned bits form at bottom of pan, 7–8 minutes. Transfer to a medium bowl. Add brown sugar. Using an electric mixer, beat until well combined and mixture resembles wet sand, 2–3 minutes. Add eggs and vanilla; beat until fluffy and well combined, about 2 minutes. Add dry ingredients; beat until smooth (batter will be thick). Using an offset or regular spatula, evenly spread batter in prepared pan.
Bake blondie until golden brown, edges pull away from sides of pan, and a tester inserted into the center comes out with a few moist crumbs attached, 20–25 minutes. Let cool completely in pan on a wire rack.
Preheat oven to 350 degrees. Line a rimmed baking sheet with parchment paper. Spread peanuts over sheet in an even layer. Bake, stirring frequently, until golden brown and fragrant, 5–7 minutes. Set aside.
Stir sugar and 1/2 cup water in a large saucepan over medium-low heat until sugar dissolves. Increase heat; boil without stirring, occassionally swirling pan and brushing down sides with a wet pastry brush, until caramel is deep amber, 12–15 minutes. Add honey; return to a boil, stirring often, about 1 minute longer. Add margarine; stir until blended. Add pareve cream (mixture will bubble vigorously); whisk until smooth. Stir in peanuts and pretzels. Pour over cooled blondie. Chill until cool, about 30 minutes.
Run a knife around short sides of pan to release blondie. Using parchment-paper overhang, lift from pan. Cut lengthwise into 4 strips. Cut each strip crosswise into squares.