Preheat oven to 350°F (175°C). Line a 9×9-inch pan with foil or parchment, leaving an overhang on two sides to lift the brownies out later, and spray the lining lightly with nonstick cooking spray (do not use a spray that contains flour).

Place the 4 ounces (113 g) unsweetened baking chocolate and ¾ cup (170 g) unsalted butter in a large microwave-safe bowl.

Melt the chocolate and butter in the microwave in 30-second increments, stirring well between each interval, until completely melted and smooth (about 2 minutes total, depending on your microwave). Alternatively, melt in a heatproof bowl set over simmering water (double boiler), stirring until smooth.

Stir the 1 ¾ cups (350 g) granulated sugar into the melted chocolate mixture until combined.

Add the 3 large eggs, one at a time, stirring well after each addition until the mixture is smooth and glossy.

Stir in 1 teaspoon vanilla extract, 2 tablespoons (10 g) unsweetened cocoa powder, and ½ teaspoon salt until evenly incorporated.

Carefully fold in 1 cup Gluten-Free 1-to-1 Baking Flour until just combined; do not overmix. (Optional: fold in any gluten-free candy or chopped cookies now if using.)

Pour the batter into the prepared pan and spread it evenly with a spatula.

Bake at 350°F for about 22–25 minutes. To check doneness, insert a toothpick about 1 inch from the edge of the pan — it should come out with just a few moist crumbs. For fudgier brownies, err on the side of slightly underbaking. If using a 9×13-inch pan for thinner brownies, the total baking time will be shorter, so begin checking several minutes earlier.

Cool the brownies completely in the pan on a wire rack. Use the foil or parchment overhang to lift them from the pan, then cut into squares.

Store cooled brownies in an airtight container for up to 4 days, or freeze for up to one month.