Use a fork to mix together the chocolate wafer crumbs and butter. Spray the cups of a 6-muffin tin with a vegetable spray. Line the cups with strips of parchment or waxed paper, cut so that they are as wide as the diameter of each cup and long enough to overhang the sides (you'll need this overhang to remove the tarts). Use the bottom of a small glass to press the crumbs over the bottom and sides of the muffin cups, building the sides up to only about 1 1/2 inches. Place a raspberry in the center of each crust and with a pastry bag or small plastic bag with the corner snipped off, fill the tarts with the truffle mixture. Smooth the tops and refrigerate until set, about 2 hours.;
In a saucepan, bring the cream just to a simmer over low heat. Pour the cream over the chocolate in a bowl and let stand about 10 minutes to melt the chocolate. Add the vanilla and stir until smooth. Set aside to cool for 1 hour at room temperature. Then beat the chocolate at medium speed until it gets thick and light colored. Spread over the bottom of a baking dish and smooth the top. Refrigerate about 2 hours, until firm.
Pour the cocoa powder onto a deep plate or shallow bowl. Use a melon baller or small ice cream scoop to scoop out balls of chocolate; place them on the plate with the cocoa powder and roll between 2 forks to completely coat with the cocoa powder. Then use the forks to carefully transfer them to a parchment or waxed lined baking sheet.