Preheat oven to 350°F.
Spray 8"x 12" or 9" x13" baking sheet with nonstick spray. Place parchment on top and set aside.
Sift flour, salt, and cocoa powder together in medium bowl.
Separate egg whites and egg yolks into separate bowls.
Whip egg whites on high until it forms soft peaks. Scrape into separate bowl and clean mixing bowl.
Whisk egg yolks, sugar, and vanilla together.
Melt butter and chocolate together in microwave for 60 to 90 seconds until chocolate and butter have melted. Whisk to combine.
Whip egg yolk mixture on high for 5 minutes until it reaches ribbon stage and you can draw a figure 8 that holds for approx 5 seconds.
Whisk in chocolate mixture.
Fold in flour mixture.
Fold in egg whites.
Pour batter into prepared pan and bake for 10-12 minutes until just firm to touch.
While S'mores cake batter is baking, place a new piece of parchment on the counter and dust evenly with caster sugar.
Remove cake from oven, immediately flip onto caster sugar-lined parchment and peel away parchment cake was baked on.
Score cake on short side 1/2" from end closest to you, fold parchment paper over this end and carefully roll cake. Leave in parchment and set aside until cool.