2 tablespoons unsalted butter, cut into small pieces
3 cups semisweet chocolate chips (16 ounces)
1 can (14 ounces) sweetened condensed milk
1/2 teaspoon pure vanilla extract
1/8 teaspoon fine salt
2 1/2 cups roughly chopped miniature pretzels
Coat an 8-inch square baking pan with cooking spray and line with parchment paper, leaving a 2-inch overhang on all sides.
Place butter, chocolate chips, condensed milk, vanilla, and salt in a medium heatproof bowl set over (not in) a pot of barely simmering water. Stir occasionally until chocolate just melts and mixture is combined and warm but not hot, 8 to 10 minutes. Remove from heat and stir in 2 cups pretzels. Transfer mixture to pan and smooth top; press 1/2 cup pretzels on top. Refrigerate until set, 2 hours (or up to overnight, wrapped). Using parchment, lift fudge from pan and cut into 36 squares.
Fudge can be stored in an airtight container, up to 2 weeks.
Swap It: Use trail mix, your favorite chopped candy, or crushed cookies in place of pretzels.