Preheat oven to 375°F. Lightly grease a 9x13" rimmed baking pan, line it with wax paper, then grease paper; set aside.
Sift together flour, cocoa, baking powder, salt; set aside.
In a medium-size bowl, beat eggs with an electric mixer at high speed until eggs are thick and lemon colored. Force brown sugar trhough a strainer to get rid of hard lumps. Gradually add borwn sugar and granulated sugar to eggs, while beating egg mixture on high speed. Continue beating, scraping bowl often, until mixture is light and fluffy and falls in a thick ribbon. With a rubber spatula, mix in water and vanilla.
Sprinkle about a thrid of flour mixture over eggs and carefully fold together. Add another third of flour mixture and fold together. Repeat with final third of flour mixture, folding together until ingredients are well blended. Pour batter into prepared pan and spread evenly.
Bake until top springs back when gently touched, about 12 minutes. Immediately invert cake onto a clean dish towel sprinkled with about 1/3 cup powdered sugar. Remove wax paper and immediately roll cake and towel into a cylander. Let coll completely on a wire rack.
Unroll cake, spread with filling, then reroll. Moisten the outside of the cake with a little of the filling and sprinke with chocolate sprinkles or shavings. If made ahead, cover loosely with plastic wrap and refrigerate for a long as 24 hours. To serve, let cake warm to room temperature. If desired, sift additional powdered sugar over cake top and garnish with orange slices.
Beat egg white in a small bowl until soft peaks form. Gradually beat in 1/4 cup sugar until soft peaks form (again). Separately, whip 1/2 cup whipping cream and fold into egg whites. Add 1/2 cup mashed strawberries and chill filling for at least 30 minutes before using.