Preheat oven to 375°F. Line a baking sheet with parchment or a silicone baking mat. Set aside.
Brush 12 large metal french horn cones with softened butter.
On a non stick surface unroll puff pastry into approximately an 9-10 x 8 inch rectangle. Using a sharp knife or a pastry cutter, cut into ¾ inch strips. Wind pastry strips around metal cones, starting at narrow end, slightly overlapping layers, to form a spiral. (When cutting the strips, strive for even size, don't stress over exact width)
Brush puff pastry on all sides with beaten egg wash. Dust with powdered sugar. Place seam side down on pan.
Bake for 15-20 minutes or until golden and puffed. Let cool completely on metal forms.
To prepare filling: Use an electric mixer to whip cream, vanilla and cream of tarter until soft peaks form. Gradually add sugar, adjusting amount to taste.
Gently remove cooled cream horns from metal forms and fill with whipped cream, using a piping bag or a Ziploc bag with the corner cut off.
Serve immediately dusted with additional powdered sugar and fresh berries, if desired.
Store leftovers in an airtight container for up to 3 days.