Line a half sheet pan with parchment paper, spreading a bit of shortening on top, underneath and on the sides.
In a stand mixer, whip yolks on high speed briefly then switch to low, add 3 oz sugar, switch back to high until a ribbon forms. Add vanilla and Trablit on low speed. Set aside.
In a clean mixing bowl, whip whites on high speed briefly then switch to medium, add 3 oz sugar, switch to low until whites reach a stiff peak.
Sift dry ingredients together.
Fold meringue into yolks.
Fold dry ingredients into mixture.
Quickly pour out onto half sheet pan. Level with an offset spatula. Clean around the sides once with your thumb.
Bake at 400 for 9 minutes.
Roulade assembly:
Use a paring knife around corners to remove roulade from pan while hot.
Drizzle sugar on a 2nd piece of parchment and turn the cake out onto it.
Roll tightly with parchment, cool, uncoil.
Brush simple syrup mixed with Kahlua on top.
Spread a thin layer of butter cream on top.
Roll, pack tightly while rolling.
Optional: sift powdered sugar over the top before serving.