Slice the base off the pandoro with a serrated knife, saving it for later. Hollow out the loaf, leaving a 1 cm border.
For chocolate semifreddo, heat cocoa and milk in a small pan over medium heat until simmering. Whisk to a smooth paste and let cool.
In a bowl, whisk ricotta, vanilla, and 1/2 the sugar until smooth. In a separate large bowl, whisk egg whites until foamy. Gradually add the remaining sugar and whisk until soft peaks form. In another bowl, whisk cream until soft peaks form. Fold the ricotta mixture into the cream, then gently fold in the egg whites.
Divide the mixture into two bowls. Add the cooled cocoa paste and chopped chocolate to one bowl. To the other, fold in amaretto, almond extract, chopped almonds, and candied peel.
Spoon the amaretto semifreddo into the hollowed pandoro, followed by the chocolate semifreddo. Place the reserved pandoro base on top and freeze for at least 6 hours, until solid.
When ready to serve, turn the pandoro onto a serving plate. Drizzle with melted chocolate and sprinkle with chopped pistachios. Let thaw for 30 minutes before slicing and serving.