Prepare syrup: In a saucepan, combine water and honey. Bring to a boil, then remove from heat.
Melt chocolate: Add chopped couverture and cocoa to the hot syrup. Wait a few seconds before stirring to allow the chocolate to soften, then mix until smooth.
Prepare granola: Divide the granola, blending half into a powder. Combine both the powdered and whole granola in a large bowl.
Combine ingredients: Add cinnamon, cognac, and hand-broken walnuts to the granola. Pour in the chocolate mixture and mix well.
Form the trunk: Spread the mixture onto parchment paper. Shape into a roll, wrap with cling film, and freeze until set.
Coat with chocolate: Cut the frozen roll into slices. Dip each slice in melted couverture, remove excess chocolate, and allow to set on cling film.
Serve: Once the chocolate coating has set, transfer the cariocas to a serving tray.