Prep the pan. Heat the oven to 350°F (175°C). Line an 8-inch (20 cm) square pan with parchment, leaving overhang for easy lifting. Lightly grease the paper.

Melt the chocolate and butter. In a heatproof bowl, melt butter and chopped dark chocolate together over a gentle double boiler or in the microwave in 20–30 second bursts. Stir until smooth and glossy. Let cool 3–4 minutes so it’s warm, not hot.

Sweeten and whip. Whisk the erythritol (and allulose, if using) into the warm chocolate mixture. Add the eggs and yolk one at a time, whisking well after each. Beat for 45–60 seconds until slightly thick and shiny—this helps the brownies set fudgy and form a light crust.

Add flavors. Whisk in vanilla, espresso powder, and salt. The batter should smell deeply chocolatey.

Combine dry ingredients. In a separate bowl, whisk almond flour, coconut flour, cocoa powder, and baking powder (if using) until no lumps remain.

Fold together. Add dry ingredients to the wet mixture. Switch to a spatula and fold gently until just combined. Avoid overmixing. If using chocolate chips, fold them in now.

Pan and smooth. Scrape the thick batter into the prepared pan and smooth the top with an offset spatula. Tap the pan lightly on the counter to settle air bubbles.

Bake. Bake for 18–22 minutes. Start checking at 18 minutes. The edges should look set, and a toothpick inserted 2 inches from the edge should come out with moist crumbs. The center may look slightly soft—that’s good for fudgy texture.

Cool completely. Place the pan on a rack. Cool at least 1 hour before lifting out. For clean, neat slices, chill for 30 minutes after cooling, then cut with a sharp knife wiped between cuts.

Finish and serve. Sprinkle with a pinch of flaky salt if you like. Enjoy at room temperature for maximum fudginess.