To prepare the cookies, preheat the oven to 350 degrees Fahrenheit. Mix together flour, cocoa powder, and baking powder; set aside. In the bowl of a mixer, beat the butter until creamy. Add sugars, salt, vanilla, and mix. Add the eggs; beat for two minutes.
Add the dry ingredients slowly. Mix on low speed for several more minutes, until the flour is fully combined into the dough.
Roll out the dough. With a cookie cutter or knife cut out desired shapes. Bake on a cookie sheet for 25 minutes. Allow to cool.
To prepare the white chocolate mousse, melt white chocolate with two tablespoons of the heavy cream over a double boiler. Whip the heavy cream until peaks form. Add zest and vanilla. Add two tablespoons of the whipped heavy cream to the melted white chocolate and combine. Allow the white chocolate to cool for several minutes.
Fold the melted chocolate into the cream; combine well. Keep refrigerated until ready to serve.
To serve, place a shortbread cookie on a serving plate. Top with white chocolate cream and berries. Top with additional shortbread cookie. Add a dollop of cream and some more berries. Garnish with shaved chocolate (optional).