Prep the pan and oven: Heat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease the sides.

Melt butter and chocolate: In a heatproof bowl, melt 1/2 cup butter with 3 ounces unsweetened chocolate. Use short microwave bursts or a double boiler. Stir until glossy and smooth, then cool slightly.

Whisk the dry ingredients: In a separate bowl, whisk 3/4 cup fine almond flour, 1/4 cup cocoa powder, 1/2 teaspoon baking powder, and 1/4 teaspoon sea salt to remove lumps.

Beat eggs and sweetener: In a mixing bowl, whisk 2 large eggs with 3/4 cup keto sweetener until slightly thick and lightened in color, 1–2 minutes. This helps with that fudgy chew.

Combine wet ingredients: Whisk in 1 teaspoon vanilla and the melted chocolate-butter mixture until smooth.

Fold in dry ingredients: Add the dry mixture and gently fold until just combined. Do not overmix. If using, fold in 1/3–1/2 cup sugar-free chocolate chips or 1/2 cup chopped nuts.

Spread and smooth: Pour batter into the prepared pan and smooth the top with a spatula. The batter will be thick.

Bake: Bake 18–22 minutes, until the edges are set and a toothpick inserted near the center comes out with a few moist crumbs. For extra fudgy, start checking at 17 minutes.

Cool completely: Let the brownies cool in the pan on a rack for at least 45–60 minutes. They firm up as they cool. Lift out using parchment and slice with a sharp knife.

Finish and serve: Sprinkle a pinch of flaky sea salt on top if you like. Serve at room temperature or slightly chilled for maximum fudginess.