1. Preheat oven to 350°.
2. To prepare crisps, melt butter in a small skillet over medium heat; cook 3 minutes or until lightly browned, stirring occasionally. Remove from heat.
3. Lightly dab 5 phyllo sheets with half of browned butter; sprinkle each sheet with 1 1/2 teaspoons sugar. Stack layers together; gently press. Cut phyllo stack into 9 (4 2/3 x 3–inch) rectangles. Repeat procedure with remaining phyllo, browned butter, and 2 1/2 tablespoons sugar. Place rectangles on baking sheets. Bake at 350° for 5 minutes or until crisp. Remove from pans; cool completely on a wire rack.
4. To prepare fool, combine rhubarb and next 8 ingredients (through salt) in a large saucepan; bring to boil. Reduce heat, and simmer until reduced to 2 1/2 cups (about 30 minutes), stirring occasionally. Place half of rhubarb mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over the opening in blender lid (to avoid splatters). Blend until smooth. Pour into a medium bowl. Repeat procedure with remaining rhubarb mixture. Cover and chill completely.
5. Place whipping cream, 2 teaspoons sugar, and vanilla in a small bowl; beat with a mixer at high speed until stiff peaks form. Gently fold in sour cream. Fold whipped cream mixture into chilled rhubarb mixture.
6. Place 1 phyllo crisp on a dessert plate. Layer crisp with 1/4 cup rhubarb mixture, 1 phyllo crisp, 1/4 cup rhubarb mixture, and 1 phyllo crisp. Repeat procedure with remaining phyllo crisps and rhubarb mixture. Serve immediately.