Preheat oven and prepare pan:
Preheat oven to 350°F (175°C). Line a 15×10-inch jelly roll pan with parchment paper and lightly grease. Dust parchment with cocoa powder.

Mix dry ingredients:
In a bowl, sift together cake flour, cocoa powder, baking powder, and salt. Set aside.

Beat egg yolks and oil:
In a large bowl, whisk egg yolks with half the sugar (3/8 cup) until pale and thick. Gradually add vegetable oil, buttermilk, vanilla extract, and espresso powder (if using).

Combine wet and dry:
Gently fold the dry ingredients into the yolk mixture until just combined.

Whip egg whites:
In a separate clean bowl, beat egg whites until soft peaks form. Gradually add remaining sugar and beat until stiff peaks form.

Fold egg whites:
Carefully fold whipped egg whites into the batter in three additions, keeping as much air as possible.

Bake:
Pour batter into prepared pan, spreading evenly. Bake 12-15 minutes, or until the cake springs back when lightly touched.

Roll cake:
While warm, invert cake onto a clean kitchen towel dusted with cocoa powder. Carefully peel off parchment paper. Starting from the short edge, roll cake and towel together. Let cool completely.

Prepare whipped cream filling:
Beat cold heavy cream, sugar, and vanilla extract until stiff peaks form. Fold in marshmallow creme if using.

Unroll and fill:
Gently unroll the cooled cake, spread whipped cream filling evenly, then roll back up without the towel.

Make chocolate topping:
Heat heavy cream until just simmering. Pour over chopped chocolate and let sit 2 minutes. Stir until smooth and glossy. Stir in corn syrup if desired.