Heat oven to 375 F.
Line a jelly roll pan or large cookie sheet (approx 15x10x1) with parchment paper.
In a medium bowl, combine flour, cocoa powder, baking powder, and salt. Set aside.
In large mixing bowl beat eggs on high speed until thick and lemon yellow (about 3 min). Slowly beat in sugar on medium speed. Add water and vanilla, and beat on low until combined. Slowly add flour mixture and beat on low until just combined.
Pour batter in pan, and spread out to corners.
Bake 15 min, or until knife inserted in center comes out clean.
When done, loosen corners of cake and turn over onto clean dish towel. Pull off parchment paper. While still warm, carefully roll up cake and towel from narrow end. Cool rolled cake on a wire rack for 30 min.
Unroll cooled cake, remove towel, and spread slightly softened vanilla ice cream over cake, staying 1/4″ from the edge. Re-roll the cake, wrap in plastic wrap, and store in the freezer.
For the chocolate sauce, in the top of a double boiler, over simmering water, melt the butter, chocolate, and sugar until combined and glossy.
Increase heat, add salt and heavy cream. Bring to a boil, then immediately reduce to simmer. Simmer on low for 2 hours, stirring occasionally.
Remove from heat and add vanilla. Let cool to room temperature and store in refrigerator until needed.
Slice and eat, drizzled with chocolate sauce.