For hot cocoa, in a small saucepan, mix cocoa, sugar and water until smooth. Bring to a boil; cook, stirring constantly, 2 minutes. Stir in milk until blended; transfer to a shallow bowl. Cool completely.
For tiramisu, in a heatproof bowl of a stand mixer, whisk egg yolks and 1/2 cup sugar until blended. Place over simmering water in a large saucepan over medium heat. Whisking constantly, heat mixture until a thermometer reads 160°, 2-3 minutes.
Remove from heat. With the whisk attachment of a stand mixer, beat on high speed until thick and pale yellow, about 5 minutes. Add mascarpone; beat on medium speed until smooth, scraping down sides of bowl as needed.
In another bowl, beat cream until it begins to thicken. Add remaining sugar; beat until soft peaks form. Fold whipped cream into mascarpone mixture.
To assemble, spread a third of the cream mixture into a 13x9-in. baking dish. Quickly dip half of the ladyfingers halfway into cooled cocoa; arrange over cream. Repeat layers. Spread with remaining cream mixture. Sprinkle with chocolate chips and cinnamon. Refrigerate, covered, at least 8 hours or overnight.
Editor's Note This recipe was prepared with Alessi brand ladyfinger cookies.