Sift the flour, cocoa powder and baking soda into a medium bowl, then whisk in the salt.
In a large bowl, whisk the sugar and eggs until slightly lightened, about 30 seconds.
Whisk in ½ cup espresso, sour cream, butter and vanilla. Add the flour mixture and whisk gently until just combined.
Pour the batter into the steamer.
Cover and heat on high until the water boils.
Reduce to low and steam, covered, until the center of the cake is just firm to the touch, about 23 minutes.
Turn off the heat and remove the lid.
Let stand until the cake pan is cool enough to handle.
Transfer the pan to a wire rack, then run a paring knife around the edge of the cake to loosen.
Let cool completely.
Invert the cake directly onto the rack, lift off the pan and remove and discard the parchment.
Re-invert onto a serving plate.