Nonstick vegetable oil spray
½ cup (1 stick) unsalted butter or margarine
¾ cup granulated sugar
½ cup (packed) light brown sugar
½ teaspoon kosher salt
1 teaspoon vanilla extract
¼ teaspoon baking soda
4 ounces bittersweet chocolate, melted
½ cup unsalted, roasted nuts (such as almonds, pecans, and/or pistachios), chopped
¼ cup seeds (such as flax, sesame, and/or sunflower)
½ teaspoon flaky sea salt
A candy thermometer
Lightly coat a large rimmed baking sheet with nonstick spray. Combine butter, granulated and brown sugars, kosher salt, and 2 Tbsp. water in a large heavy saucepan fitted with candy thermometer. Cook over medium-high heat, swirling pan occasionally, until thermometer registers 300° (toffee should be a deep golden brown), 7–9 minutes.
Remove toffee from heat and carefully stir in vanilla. Sprinkle baking soda evenly over surface and stir just until incorporated (be careful not to overmix). Quickly scrape mixture onto prepared baking sheet and tilt from side to side to spread mixture slightly; let cool 10 minutes.
Drizzle chocolate over toffee and spread evenly with an offset spatula or large metal spoon. Scatter nuts and seeds over top, then sprinkle with sea salt. Let sit until chocolate is firm, at least 2 hours. Break into large shards.
Do Ahead: Toffee can be made 3 days ahead. Store airtight at room temperature.