Preheat your oven to 175°C (350°F) and line a 20cm square baking tin with parchment paper, leaving an overhang on two sides for easy lifting later. The paper should look crisp and clean against the metal.

Halve and scoop the avocados into a large mixing bowl. Mash them thoroughly with a fork until the texture resembles a thick, lump-free paste — you'll see a smooth, pale green cream with no visible chunks remaining.

Add the melted butter and vanilla extract to the avocado paste. Stir vigorously with a wooden spoon until the mixture turns a uniform, glossy pale green and smells faintly sweet and buttery.

Crack in the eggs one at a time, beating well after each addition. The batter should become lighter in colour and noticeably airier, with small bubbles forming on the surface as you stir.

Sift together the cocoa powder, almond flour, erythritol, baking powder, and salt in a separate bowl. This prevents lumps — you'll see a fine, dark powder with no clumps remaining in the sieve.

Fold the dry ingredients into the wet mixture in three additions, using a gentle hand until just combined. The batter should be thick, dark, and glossy, with a rich chocolate aroma filling your kitchen. Stop folding as soon as no streaks of flour remain — overmixing will make the brownies tough.

Stir in the sugar-free chocolate chips if using, reserving a handful to scatter on top. You'll hear a gentle clinking sound as the chips fold into the thick batter.

Pour the batter into the prepared tin and spread it evenly with a spatula. The surface should look smooth and level, with the chocolate chips peeking through here and there. Tap the tin gently on the counter to release any air bubbles — you'll see a few small burps rise to the surface.

Bake for 22-25 minutes until the edges look set and slightly cracked, but the centre still wobbles when you gently shake the tin. A skewer inserted 2cm from the edge should come out clean, but the centre will still look slightly wet — this is the secret to fudgy brownies.

Cool completely in the tin on a wire rack for at least 30 minutes before lifting out and slicing. The brownies will firm up as they cool, and the chocolate aroma will intensify into a deep, rich scent. For cleaner cuts, chill them in the fridge for an hour before slicing.