Jelly: Whisk together coffee and sugar in a 1-quart saucepan. Mix in gelatin. Bring to a boil and whisk constantly to dissolve gelatin. Pour mixture in a bowl and refrigerate for several hours to overnight to fully set.
When ready to prepare cake, grease and flour a jelly roll pan. Beat eggs on medium speed until thick and lemony colored. Gradually add the sugar.
Mix in coffee and vanilla. Add flour, baking powder and salt.
Pour into prepared jelly-roll pan and bake at 375 for about 12 minutes. Cake should be done but not overly baked.
Roll cake up in towel. Cool completely. Unroll add jelly, roll back up chill. Sprinkle with powdered sugar if desired.