Pre-heat oven to 350° (180C). Lightly grease a 10x15 inch (25x38 cm) jelly roll pan or cookie sheet (I used a cookie sheet) line it with parchment paper, make sure it is about 2 inches (4 cm) wider on all sides of the sheet. Then lightly grease and flour the paper. Set aside.
In a medium bowl whisk together the flour, cocoa, baking powder, baking soda and salt. Set aside.
In a large bowl beat eggs until light and fluffy about 3 minutes, continue to beat while slowly adding the sugar, then add the milk and vanilla and beat well for 1 minute.
Add the flour mixture a little at a time and beating on low speed, when all the flour has been added increase the speed to medium and beat for 1 minute.
Spread the batter evenly on the prepared pan, bang the pan a few times to release the air bubbles. Bake for approximately 12-15 minutes, do not over bake. Check for doneness by lightly touching the cake in the middle with the tip of your finger if it springs back it's done. Immediately remove the baked cake with the parchment paper from the cookie sheet and roll up with the paper still attached to the cake. Place it on a wire rack to cool.
Unroll the cake, removing the parchment paper as you unroll, then spread it with a little more than ½ of the buttercream, roll it up again and cover the outside of the roll with the remaining buttercream. Place on a plate or cookie sheet and refrigerate for approximately 10-15 minutes. Remove from the fridge, place the roll on a wire rack, on a cookie sheet and pour the chocolate glaze (if the glaze is too firm then heat gently until thinner) evenly over the roll. Refrigerate approximately 45-60 minutes before serving. Enjoy!
In a medium bowl add the chopped chocolate and corn syrup.
Heat the cream until very hot but not boiling then pour over the chocolate, let sit 1-2 minutes then stir until melted and smooth. Let cool until warmish.
For room temperature ingredients, remove from the fridge 45-60 minutes before using.Any leftover cake should be covered and stored in the fridge. It will keep for up to 3 days.You can also freeze the cake, firm up the chocolate by refrigerating the cake for approximately 30-60 minutes, then wrap it in plastic then again in foil, place in a freezer safe bag, it will keep for up to two months in the freezer.