Sprinkle dry yeast into 1-1/2 cups warm water; stir to dissolve.
Set aside until mixture begins to foam, 5 to 10 minutes.
Sift flour into a large bowl.
Make a well in the sifted flour and slowly add yeast mixture.
Mix thoroughly with a spoon.
Place dough on a floured surface and knead about 3 minutes until smooth and elastic.
Place dough in large bowl and cover with a damp cloth.
Let stand in a warm place 3 or 4 hours until doubled in bulk.
Combine filling ingredients in a medium bowl, mixing well; set aside.
Place dough on a floured surface and knead again until smooth and elastic.
Use a rolling pin to roll out to a very thin 30" x 12" rectangle.
Spread 1/6 of the filling over center third of dough.
Fold one side over filling.
Spread 1/3 of the remaining filling on top of folded section.
Fold remaining side over filling.
Turn dough so the last folded edge faces you.
Roll out again into a 18" x 12" rectangle.
Repeat folding steps with remaining filling.
Then fold dough in half and in half again to make a 6-inch square.
Spread shredded sweet cucumber on an 8-inch square of greased waxed paper.
Place dough on cucumber.
Let stand 30 minutes.
Pour 4 cups boiling water into a wok or large pot.
Place cake in a steamer.
Cover steamer and place over boiling water.
Steam over high heat 35 to 40 minutes, adding more boiling water as needed.
Remove from heat and let cool 3 to 5 minutes.
Remove cover slowly as a quick cold draft may cause cakes to collapse.