Preheat oven to 375°F. Spray a 15x10” jelly roll pan with cooking spray, then top with a sheet of parchment paper that fits inside the pan. Spray lightly with cooking spray and dust with a bit of flour (or use a baking spray that has flour in it). Set aside.
Combine flour, baking powder and salt in a small mixing bowl, whisking well.
In a larger mixing bowl, add egg yolks and beat with a mixer on medium speed for several minutes, or until thick.
Add 1/2 cup of sugar and beat until well combined. Add lemon zest and beat until combined.
Add half the flour mixture and beat until just combined. Add half of the lemonade, and beat until just combined. Repeat with remaining flour mixture and lemonade.
In a separate mixing bowl, beat egg whites and remaining 1/4 cup sugar on high speed for 3-5 minutes, or until stiff peaks form.
Add egg white mixture to the cake batter and fold in gently.
Spread evenly in prepared baking pan, and bake for 18-20 minutes.
While the cake is baking, lay out a thin kitchen towel on the counter and sprinkle generously with about 1/3-1/2 cup of powdered sugar.
When the cake is finished baking, immediately invert the cake onto the prepared towel. Peel off the parchment paper and roll the cake up, starting with the narrower end.
Place cake, seam side down, still in the towel, and let it cool completely at room temperature.
Stir together cool whip and remaining half of the lemon zest.
Gently unroll the cake, and spread cool whip on top of the unrolled cake, leaving a small 1/2” boarder around the edges.
Gently roll the cake back up, tightly, and without the towel this time. If some filling spills out, that’s okay.
Cover with plastic wrap and refrigerate for at least 30 minutes.
Before serving, dust with powdered sugar and sprinkle with lemon zest.