An updated riff on the classic icebox cake, this chilled springtime dessert pairs crisp sugar cookies with stewed rhubarb, then covers it all in lightly sweetened whipped cream. Some time in the refrigerator allows the flavors to meld and the cookies to soften, for a lovely, fork-friendly treat.
To make the cookies: Preheat the oven to 300°F. Lightly grease two baking sheets, or line with parchment.
In a small bowl, combine the vanilla, baker's ammonia (or baking powder) and salt. Stir to dissolve the ammonia; the salt won't fully dissolve.
Place the vanilla mixture, sugar, and butter in a large mixing bowl and beat with an electric mixer until smooth. Add the flour and mix to combine. The dough will seem dry at first; continue mixing until the dough comes together.
Scoop the dough with a teaspoon cookie scoop and place on the prepared baking sheets, leaving 1 1/2" between portions. Press the tops down with the bottom of a glass dipped in sugar to make flat cookies about 1/4" thick. Bake for 20 to 25 minutes, until the cookies are very light golden brown. Remove from the oven and transfer to a rack to cool.
For the filling: Place the rhubarb, 1/2 cup of the sugar, the nutmeg, and orange zest in a medium saucepan and bring to a boil. Reduce the heat and simmer for 15 to 20 minutes, until some of the liquid cooks away.
Combine the remaining 1/4 cup sugar and ClearJel and stir into the mixture. Return to a boil and cook for 1 more minute, until the sugar dissolves and the mixture thickens. If using cornstarch, combine it with the remaining sugar and 2 tablespoons of cold water, stirring until no lumps remain. Stir this mixture into the simmering rhubarb and cook until the filling returns to a boil, thickens, and is no longer cloudy. Remove from the heat and cool completely before assembling the cake.
To make the whipped cream: Place the cream in a chilled mixing bowl with the sugar. Whip (slowly at first, until the cream begins to thicken, then more vigorously) until the whisk's tracks are visible in the bowl. Add the vanilla and continue beating until the cream is stiff but not curdled.
To assemble the cake: Spread a tablespoon of rhubarb filling on top of a cookie, then place another cookie on top. Continue this process until you have four cookies stacked up. You can coat each stack of 4 cookies with whipped cream to make individual desserts, or lay the stack on its side on a large serving plate. Repeat the process until you've used all the cookies. For one large cake, you can lay the stacks side by side, or form them into a circle or any other shape you like.
Cover the entire cake on all sides with whipped cream, taking care that no cookies are left uncovered. Chill for at least 4 hours (or overnight) before serving. Garnish with fresh strawberries, if desired.
Refrigerate, with cut sides covered with plastic, for up to 1 week.
