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1
Preheat oven to 375 degrees F. Spray one 9 by 17-inch sheet tray with vegetable spray, line with parchment paper and spray again.
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2
In a mixer, using the whip attachment, whip the egg yolks and the sugar until doubled in volume and pale yellow; add vanilla extract.
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3
Sift together the flour, cocoa powder, and baking powder and powdered sugar twice.
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4
Slowly add the dry ingredients to the mixing bowl, running at slow speed, until fully incorporated.
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5
In a clean and greaseless bowl, whip the egg whites until firm but not dry.
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6
Incorporate a fifth of the whipped egg whites into the cocoa mixture and mix until well incorporated.
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7
Fold the remaining egg whites into the cocoa mixture in 3 batches and immediately spread onto a large baking sheet using an offset spatula, being careful not to overwork the dough.
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8
Bake for 6 to 8 minutes.
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9
Immediately run the tip of a knife around the edge of the cake to loosen it from the sides of the sheet tray then slide it (with the parchment paper) onto a cool surface.
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10
Trim dry edges of the cake.
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11
Vanilla Cream Filling: With an electric mixer, beat together the butter and the vegetable shortening until blended.
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12
Turn the mixer down to its lowest setting and gradually add the confectioners' sugar.
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13
Turn the mixer back up and beat at high speed until light and fluffy, about 5 minutes.
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14
Gradually beat in the vanilla and corn syrup until the filling is the consistency of sour cream, about 2 minutes more.
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15
Using a sharp knife, cut the cake in half lengthwise.
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16
Using an offset spatula, spread a 1/8-inch thick layer of vanilla cream filling over half of the cake and cut into 5 equal sections per half.
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17
Roll up each section tightly (peeling off parchment paper as you roll) and place in the freezer for 30 minutes.
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18
Melt the bittersweet chocolate in the top of a double boiler and add cocoa butter or vegetable oil.
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19
Place the rolls 1 at a time directly in the double boiler of melted chocolate.
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20
Coat each roll completely in the chocolate.
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21
Using two forks, carefully transfer the rolls to a parchment-lined tray, allowing any excess chocolate to drip back into the double boiler.
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22
Continue with the remaining rolls, pouring more of the chocolate onto the plate as needed.
