Place a rack and an oven thermometer in middle of oven; preheat to 225°. Using your fingers, gently break up ½ g cannabis flower into raisin-size pieces and spread out in an even layer on a parchment-lined baking sheet. Bake, gently tossing every 10 minutes or so to avoid burning, until weed turns brownish green (indicating it has decarboxylated), 10–20 minutes if using old or lower-quality weed, 15–30 minutes for cured high-quality weed, or 45–60 minutes for anything recently harvested and still damp. Let cool on baking sheet. Increase oven temperature to 350°.

Pour water into a medium saucepan to come about 2" up sides. Place a medium heatproof mixing bowl over pot (bowl should not touch water) and set over medium-low heat. Heat ½ cup (1 stick) unsalted butter and 1 Tbsp. water until butter is melted. Crumble weed with your hands directly into butter. Discard parchment paper; reserve baking sheet. Reduce heat to low and cook, checking occasionally with an instant-read thermometer and adjusting heat as needed to ensure butter temperature does not exceed 190°, 1 hour.

Meanwhile, spread out ⅔ cup coarsely chopped pecans or walnuts on reserved baking sheet and toast, tossing halfway through, until slightly darkened in color and fragrant, 8–10 minutes.

Lightly coat an 8x8" baking pan with nonstick vegetable oil spray or room-temperature unsalted butter, then line with parchment paper, leaving generous overhang on 2 sides. Lightly coat parchment with nonstick spray or butter.

Pour cannabis-infused butter through a fine-mesh sieve into a large heatproof bowl. Immediately stir in 1 cup (84 g) Dutch-process cocoa powder followed by 1¼ cups (250 g) light brown sugar, 2 tsp. vanilla bean paste or vanilla extract, 1 tsp. instant espresso powder (if using), and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt with a rubber spatula. Add 2 large eggs, one at a time, whisking until incorporated after each addition. Continue to whisk vigorously until mixture is glossy, lightened in color, and slightly increased in volume. Stir in ⅔ cup (84 g) all-purpose flour, then pecans. Scrape batter into prepared baking pan; smooth surface. Sprinkle flaky sea salt over if desired.

Bake brownies until set on top and a tester inserted into the center comes out with just a few moist crumbs attached, 25–28 minutes. Remove from oven and tap on stovetop a couple times to deflate slightly (this boosts their chewy-fudgy texture). Let cool 15 minutes, then chill until completely cool, about 45 minutes.

Using parchment paper overhang, remove brownies from pan and cut into 16 pieces (about 6¼ mg THC per piece), 12 pieces (about 8⅓ mg THC per piece), or 9 pieces (about 11 mg THC per piece). Do Ahead: Weed brownies can be baked 4 days ahead. Store tightly wrapped in a clearly labeled airtight container at room temperature, or freeze up to 1 month.