In a large bowl, beat butter until fluffy. Stir in sugar, salt and vanilla, using a spatula.
In a small bowl sift the flour and cocoa; add to the butter mixture. Stir until ingredients are combined and a soft dough forms.
Divide dough in half, place dough on opposite sides of the cookie sheet. Using your fingers, pat each piece of dough into a 6 inch round. Using a sharp knife score each dough round into 8 slices. Place entire cookie sheet in freezer for 25 minutes.
Preheat oven to 375 degrees. Remove cookie sheet from freezer and allow to sit for 5 minutes. Prick dough all over with a fork and bake for approximately 15 minutes until center is dry to the touch.
Remove from oven, place cookie sheet on a cooling rack for 10 minutes. Slice shortbread into wedges. Remove wedges from baking sheet and cool completely on racks.
When cooled, spread a thick layer of jam on each wedge.
In a small bowl, combine white chocolate and shortening, place in microwave, cook until melted; stir until smooth. Place chocolate in a pastry bag with a small tip and pipe "garland" on each "tree wedge". Allow cookies to sit, until chocolate has hardened. Place in an air tight container with waxed paper between layers.