Preheat oven to 325˚F (165˚C). Line a quarter sheet pan (or 12 x 8 inches cake pan) with a parchment paper. Make sure the sides are covered as well.
In a microwave safe bowl, add the milk and microwave for 20 seconds until warm. Add instant coffee and stir until dissolved. Set aside to cool.
In a large mixing bowl, sift together cake flour, 30 grams sugar, baking powder and salt. Set aside.
In a large mixing bowl with an electric mixer fitted with a whisk attachment, beat the egg whites on medium-low speed until foamy, about 1 minute. Add cream of tartar. Turn on the speed to medium and whip until the bubbles are small & uniform in size. Slowly add 50 grams of sugar a little bit at a time. Turn the speed to high and whip until stiff peaks form. (Image 1)
In the bowl of flour, create a well in the center. Add egg yolks, vegetable oil, vanilla extract and the cooled coffee milk. Stir to mix with a spatula until no lumps, but do not overmix.
Add all the egg whites into the flour batter and gently fold until just incorporated (do not stir). Pour batter into the prepared sheet pan. Spread out the batter into an even layer using an offset spatula. Bang the pan on the counter a few times to remove any large air bubbles.
Bake for 13 – 16 minutes or until the cake springs back when touch gently in the center and a toothpick inserted into the cake comes out clean. Remove from oven.
Carefully lift the cake to a cooling rack and peel the parchment paper off the sides of the cake (leave the bottom part for now). Let cool for 5 minutes. (Image 3)
Place a new parchment paper over the cake. Invert the warm cake onto the new parchment paper backed by a hard surface like a cutting board. Gently peel off the entire parchment paper from the cake. Now, this side is the outside of the swiss roll. Let cool for 3 minutes.
Get another parchment paper. Place on top of the cake and invert the cake to this new parchment paper one more time. Carefully peel the older parchment paper off the cake. It’s normal if it peels off a thin firm of cake. This side is the inside of the swiss roll.
Take a sharp knife and diagonally slice off ½-inch cake from one side of the cake (shorter end). (Image 4) This is the seam side and it will seal the Swiss roll better when rolled up.
On the other side of the cake (shorter end), lightly score two lines using the sharp knife (do not cut through). Each line should be about ½-inch apart. (Image 5) This will help the cake to roll better. Start rolling from this end along with the parchment paper. The cake should be slightly warm and that will prevent the cake from cracking. Set the rolled cake on a cooling rack to cool completely, about 45 minutes. (Image 6)
When the cake is cooled, prepare the cream filling. In a small microwave bowl, add the cold water and sprinkle the gelatin over. Set aside to bloom for 5 minutes. Microwave the mixture for 5 – 7 seconds, then stir with a spoon until the gelatin is completely dissolved. Set aside to cool slightly. The mixture should remain in liquid form when being used. If it firms up, microwave it again.
In the meantime, in a large mixing bowl with an electric mixer fitted with a whisk attachment, whisk the heavy cream until frothy. Add sugar & a pinch of salt. Increase the speed to medium-high and whisk until the whisk leaves trails in the cream. Turn off the mixer. Scoop 2 heaping tablespoons of partially-whipped cream into the bowl of gelatin. Mix well. With mixer running on low speed, pour the gelatin mixture into the rest of the cream. Turn the speed up to medium-high and whisk until stiff peaks just formed. Do not overwhip.
To assemble, carefully unroll the cake. Spread an even layer of whipped cream over the cake by leaving 1-inch space empty near the seam side (the side where cake was trimmed off). (Image 7) Roll up the cake gently by using the parchment paper to help push the cake along. Make sure not to apply too much pressure. Use the same parchment to wrap the swiss roll tightly. The seam side of the Swiss roll should be at the bottom. Place the swiss roll on a baking pan or baking sheet. To hold the roll shape better, apply slight pressure on both sides with the help of bottles. (Image 8) That way, the roll will stay round and not oval. Chill in the fridge for at least 2 hours.
Right before serving, unwrap the Swiss roll. Using a sharp serrated knife, trim off ½-inch on both ends for better presentation. Slice and serve.