In a small saucepan over low heat, simmer the champagne, whisking occasionally, until it reduces down to 4 Tablespoons (1/4 cup). This usually takes around 20 minutes, but the time varies. Place reduced champagne in the refrigerator until completely cool.

Preheat oven to 350°F (177°C). Grease a 10×15-inch baking pan with nonstick spray or butter, then line it with parchment paper so the cake seamlessly releases in step 7. Spray or grease the parchment paper too. We want an extremely nonstick surface for this cake roll.

Whisk the cake flour, baking powder, and salt together in a medium bowl. Set aside.

Using a hand mixer or a stand mixer fitted with whisk attachment, beat the egg whites and 1/4 cup sugar together on high speed for 4-5 minutes or until stiff peaks form. Transfer to another bowl. Using the same mixing bowl you just had the egg whites in (no need to clean it!), add the egg yolks, remaining sugar, 2 Tablespoons reduced champagne, and vanilla extract. Beat together on high speed for 3-4 minutes or until light in color.

Add half of the whipped egg whites into the egg yolk mixture. Beat on low speed for 10 seconds. Repeat with remaining egg whites and beat on low for 10 seconds. Finally, add the flour mixture and beat on low until the batter is completely combined. Do not overmix.

Spread batter evenly into prepared pan. Bake for 15 minutes or until the cake springs back when poked with your finger.

As the cake bakes, place a piece of parchment paper (larger than the cake) or a thin kitchen towel flat on the counter. Sprinkle with 1 cup of confectioners’ sugar. Once the cake comes out of the oven, immediately invert it onto the parchment/towel. Peel off the parchment paper that was on the bottom of the cake as it baked. Starting with the narrow end, begin rolling the cake up with the parchment/towel. Do this slowly and gently. The cake will be warm. Allow the cake to cool completely rolled up in the parchment/towel. Place in the refrigerator to speed it up, about 2 hours.

Remove the cake roll from the refrigerator and allow to sit on the counter for a few minutes to warm up as you prepare the whipped cream.

Using a hand mixer or a stand mixer fitted with a whisk attachment, whip the heavy cream, confectioners’ sugar, 1 Tablespoon reduced champagne, and vanilla extract on medium-high speed until medium to stiff peaks form, about 2-3 minutes.

Gently and slowly unroll the cake. Brush with remaining reduced champagne. Dust with cocoa powder, if desired. Spread whipped cream evenly on top, leaving about a 1/2 inch border around the cake. Gently roll the cake back up, without the parchment/towel this time. Loosely cover with plastic wrap and refrigerate for 20 minutes before slicing and serving. Dust with more confectioners’ sugar, if desired.

Cover leftover cake and store in the refrigerator for up to 3 days.