1½ cups fresh strawberries (8 oz.), rinsed, hulled and coarsely chopped*
4 large egg whites
1¼ cups sugar
3 sticks (1½ cups) unsalted butter, at room temperature
*To make the strawberry puree, process partially thawed frozen strawberries with a bit of sugar in a blender or food processor until smooth. If using sweetened frozen strawberries, no additional sugar is necessary.