The cake layer can be baked and frozen; the assembled mini cakes can be chilled up to 2 days ahead.
Prepare Chocolate Cake batter; pour into a greased and floured 15- x 10-inch jellyroll pan.
Bake at 350° for 22 to 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes. Invert onto a parchment or wax paper-lined rack; cool.
Lift parchment paper and cake layer together, and place on a cutting board. Cut 12 (3-inch) circles with a round biscuit cutter. (Reserve trimmings for another use.)
Prepare mixture for Chocolate-Praline Truffles.
Spread truffle mixture in a 1/4-inch-thick layer over top of 6 cake circles, reserving remaining truffle mixture; top each with another cake circle. Cover and chill 1 hour.
Spread remaining truffle mixture over top and sides of cakes.
Combine Praline Almonds and 1/4 cup chopped pistachios; roll cake edges in mixture.
Spoon White Chocolate Buttercream Frosting into a small heavy-duty zip-top plastic bag; seal. Snip a 1/2-inch hole in 1 corner of bag. Pipe a frosting mound evenly on top of each cake; sprinkle evenly with shaved white chocolate.
Draw 6 (3-inch) circles on a sheet of parchment paper. Turn over, and line a baking sheet with paper, making sure pattern shows through.
Microwave candy coating squares in a 2-quart microwave-safe bowl at MEDIUM (50% power) 3 minutes or until melted, whisking every 30 seconds. Spoon into a small heavy-duty zip-top plastic bag; seal. Snip a tiny hole in 1 corner of bag.
Drizzle a triangle inside each circle, forming a tree shape; randomly drizzle coating inside each triangle. Sprinkle with remaining 1/4 cup chopped pistachios and sparkling sugar. Freeze 30 minutes or until firm. Peel from paper. Gently insert a tree into each frosting mound. Store in refrigerator up to 2 days.