Prepare sponge cake.
Spread evenly with blackberry jam.
Roll up; let stand until ready to use.
Sprinkle gelatin over cold water; let stand 3 minutes to soften.
Prepare custard in double boiler; stir in soft ened gelatin until dissolved.
Place top of double boiler in bowl filled with ice.
Let stand 5 minutes; stir occasionally.
Cut rolled sponge cake into 10 even slices.
Place 1 slice in center of oiled 2-quart mold; arrange 3 slices evenly around side of mold.
Slowly pour in custard to half the depth of the 3 slices.
Place another slice in center of custard; arrange 3 slices around side between first 3 slices.
Pour in more custard to half the depth of last 3 slices added.
Place another slice in center of custard.
Cut remaining cake slice into thirds, so that each third has curved side; place around side of mold, straight-edge-down.
Pour in remaining custard.
Chill several hours or until firm.
Unmold and cut in wedges to serve.
Rolled sponge cake can be made day ahead and wrapped in plastic wrap, if desired