Crust
Graham Crackers in a food processor, or smash in a sealed plastic bag until crumbs.
Stir together the Graham Cracker crumbs, Sugar and Butter, and mix until all the crumbs are moist.
Press the mixture into the bottom and up the sides of an un-greased 9-inch fluted tart pan with a removeable base. A spoon and measuring cup work well. Place in the freezer to set.
Filling
Place the White Chocolate Chips, Cream and Butter in a microwave-safe bowl, and heat in 20 second bursts, stirring after each time, until melted. Stir until smooth. This should only take a minute or two.
Place the Blackberries inside the chilled crust, forming a single layer on the bottom. (If the blackberries seem too big, you can cut them in half).
Pour the white chocolate mixture over the blackberry layer, and smooth. Refrigerate about 3 hours, or overnight, until firm.
When the tart filling is set, decorate as desired. Slice, and enjoy!