01 -
Stir the chia seeds and warm milk in a small bowl. Let stand for 15 minutes, stirring once or twice, until thick and gel-like.
02 -
Heat the oven to 325°F (165°C). Grease an 8-inch round cake pan and line the bottom with parchment paper.
03 -
Melt the chopped chocolate and butter together in a heatproof bowl over barely simmering water or in short microwave bursts. Stir until smooth and cool for 5 minutes.
04 -
Add the chia gel, eggs, sugar, cocoa powder, vanilla, baking powder, salt, and espresso powder if using to a blender. Blend until mostly smooth, about 45 seconds.
05 -
Pour in the melted chocolate mixture and blend just until combined. Scrape the batter into the prepared pan and smooth the top.
06 -
Bake for 24-28 minutes, until the edges are set and the center has a slight wobble. Do not bake until completely firm, or the cake may turn dry.
07 -
Cool in the pan for 45 minutes. Run a thin knife around the edge, remove from the pan, and chill for at least 1 hour before slicing for the cleanest texture.