Preheat oven to 450. Spray a 12 x 17 rimmed baking sheet with cooking spray. Line with parchment paper and coat with more cooking spray. Dust with cocoa powder and tap out excess.

Combine cocoa, flour and salt in a bowl.

Combine eggs and sugar in a heat proof bowl. Bring about 2″ of water to a simmer in a medium saucepan. Set egg mixture over saucepan and whisk until sugar has dissolved and mixture is just warm to the touch.

Transfer mixture to the bowl of a stand mixer and beat on medium speed for two minutes. Raise speed to high and beat until pale and thick, about 4 minutes more.

Add dry ingredients by sifting cocoa mixture over egg mixture and using a large rubber spatula, fold dry ingredients into eggs. When almost incorporated, pour butter down side of bowl and gently fold to combine.

Using an offset spatula, spread batter onto prepared pan. Bake cake until golden brown and springy to the touch, 6-7 minutes.

While cake is baking, dust a clean kitchen towel with cocoa. When cake is finished baking run a knife along the sides of the cake. Invert onto prepared towel and remove parchment.

Starting from a long side, roll up cake into a log with the towel inside. This gives the cake a memory, so it is easier to roll up the cake when the filling is added.

Make filling:

Beat cream, powdered sugar and vanilla until soft peaks form. Unroll cake and remove towel. Spread filling evenly over cake, leaving a 1″ border around the edges. Tightly roll up starting from the long side. Twist sides of towel tightly to secure cake inside. Chill for at least 6 hours or overnight.

When ready to glaze, transfer cake to a rack and pour half of the glaze over the top and sides and spread evenly. Now spread remaining glaze over the entire cake and refrigerate until set, at least two hours. Decorate as desired.

Glaze:

Put the chopped chocolate in a bowl. In a saucepan stir the cream, milk, and sugar over moderate heat until the sugar dissolves. Pour the hot cream over the chocolate and let stand until the chocolate is melted, about 3 minutes. Whisk until smooth, then whisk in the butter, piece by piece and cool slightly.