For meringue cookies, preheat oven to 225 °F. Whip egg whites with cream of tartar until foamy. While whipping, gradually add sugar and whip on one speed lower than highest (for a tighter, more stable meringue) until whites hold a stiff peak. Whites with this much sugar will not over whip, so give it the time it needs, about 5 minutes.
Transfer meringue to a piping bag fitted with a large (#4) star tip and pipe 16 2 –inch round “cookies” on a parchment lined baking sheet. Bake meringue on center rack of oven for 1 ½ hours, then turn off oven and leave cookies in until dry (they should lift easily off the parchment). Let meringues cool to room temperature.
For raspberry fool, crush 1 cup of raspberries with 3 Tbsp of sugar and set aside. Whip cream to medium peaks and whip in remaining 2 Tbsp sugar, sour cream and orange zest. Fold in crushed raspberries roughly (they don’t have to be fully incorporated).
Immediately before serving, crumble meringue cookies into raspberry fool and spoon into serving glasses.
Prepare Meringue Cookies and Raspberry Fool (filling) as above.
To serve, spoon a dollop of raspberry fool between the two meringue cookies and place on a serving plate. Assemble immediately before serving.