The softest tender vanilla sponge cake with a vanilla cream filling and chocolate fudge or berry jam swirl! It's easy to make and without the fuss of separating eggs!
Preheat your oven to 350°F. Lightly grease a 15x10-inch jelly roll pan and line it with parchment paper. You can also use a 17x11-inch cookie sheet and the cake will be slightly thinner and bake faster.
Make the sponge batter. Place eggs in the bowl of a stand mixer fitted with the whisk attachment (or in a large bowl if using a hand mixer) and beat on high speed for 3 minutes until very frothy and doubled in volume. Add sugar one tablespoon at a time and beat for 5-8 minutes until pale, thick and tripled in volume. If you're using a hand mixer, it will certainly take 8 minutes on high speed, so don't under-whip it. The eggs should reach the “ribbon stage” where it can hold a figure “8” as it falls back onto itself when lifted with the beaters. Mix in salt.
Whisk together milk, oil, vinegar and vanilla in a small bowl and gradually pour it into the egg mixture while mixing gently on low speed.
Sift flour and baking powder into a medium bowl and whisk to blend evenly. Then, sift it again over the mixer bowl with the beaten eggs in two parts and mix it in gently on low speed until just incorporated, then finish mixing it in by hand using a large balloon whisk or wide spatula until evenly combined, scraping along the bottom of the bowl to evenly incorporate the ingredients.
Spread the batter evenly into prepared pan. Bake for 12-15 minutes if using a 10x15-inch jelly roll pan or 10-12 minutes if using a 11x17-inch sheet pan. Bake until lightly golden and it springs back when pressed gently. Do not over-bake or it will tend to crack since the surface will be dried out. Transfer pan to a wire rack to cool for a minute.
Meanwhile, lay a tea towel onto a work surface. Sift icing sugar generously over the warm cake to cover the surface and then place a large piece of parchment paper over the sugar-coated cake. Carefully but swiftly invert it onto the towel so the sugared surface is facing down. Carefully peel off the paper that was baked onto the cake. Tightly roll the cake up with the towel/parchment and let cool completely like this. NOTE: the additional layer of parchment is optional, but I find it keeps things a bit neater.
Make the filling. Combine cream, skim milk powder and sugar in a medium bowl and beat until it forms firm peaks. Mix in vanilla.
Assemble the cake. Unroll the cooled cake and first spread the jam or hot fudge sauce in a thin layer over the cake. Then, spread the cream filling over the surface leaving about ½-inch border. Gently but tightly roll up the cake with the filling and refrigerate for at least 1 hour before serving. Dust with icing sugar before slicing.