Whip 1 cup of cream to soft peaks, then refrigerate. Whisk the egg yolks with ¼ cup of the sugar, the ground candy caps, and reserve. Heat the ½ cup of cream until steaming, then whisk half into the egg yolks thoroughly, then add the other half.
Return the egg mixture to the pan and cook on low heat, whisking gently, until the mixture thickens slightly and lightly coats the back of a spoon-a minute or two. Do not over heat or the mixture will curdle.
Transfer the mixture to a bowl to cool, whisking occasionally to help cool it.
Whisk the egg whites with the remaining ¼ cup sugar until stiff peaks form. Fold the egg whites into the yolk mixture gently, then the whipped cream. Finally spoon into a loaf pan lined with plastic or individual ramekins, cover with plastic, and freeze overnight to set.
Serving
Remove the semmifreddo from the freezer, and un-mold onto a cutting board, cut off slices (you can make it super easy by dipping the knife in boiling water, or running under a flame of a burner). Put the slices on chilled plates, garnish and serve immediately.