Lipid Fusion Base: Melt butter in microwave until hot (180-190°F). Add chocolate chips, let sit 30 seconds, whisk until smooth.
The Albumin Foam: In separate bowl, combine eggs and sugar. Whip with electric mixer on high speed for 3-4 full minutes until pale, thick, and ribbony. This completely dissolves sugar.
The Batter Blend: Pour warm chocolate-butter into whipped eggs while mixing on low speed until just combined.
The Starch Fold: Sift cocoa powder, flour, espresso powder, and salt. Add to wet mixture. Fold gently with spatula 15-20 strokes until flour disappears. Do not overmix.
The Air Fryer Blast: Line 6x6 inch pan with parchment. Pour batter inside. Preheat air fryer to 325°F (163°C). Bake 16-18 minutes until edges set but center wobbles.
The Density Chill: Cool to room temperature, then refrigerate 1 hour before cutting.