Preheat oven to 325 degrees F. Line a half sheet (13x18-inch) rimmed baking tray with parchment. Spritz with nonstick cooking spray.

Prepare cake mix according to package instructions. Stir in lemon zest.

With palette knife, spread batter over prepared tray, evenly covering entire surface. Bake 25 minutes or until cake springs back lightly when touched.

Flip cake onto greased wire rack. Peel away parchment. Place greased tray or heavy cutting board over cake to compress. Cool 30 minutes.

Flip cooled cake onto clean cutting board. Trim edges with sharp knife. Slice cake width-wise into 3 rectangles, each about 5-1/4 inches wide x 11 inches long.

With small palette knife, spread 1/4 cup blueberry jam over first rectangle. Top with second rectangle. Spread with remaining jam. Top with third rectangle. Pat down gently to secure. Place cake on clean tray and freeze 1 hour or until very firm.

With sharp chef's knife, trim frozen cake to 5 inches wide x 10 inches long. Cut into 50 squares, each 1 inch in size. Freeze squares 30 minutes. Meanwhile, prepare the glaze.

Melt white chocolate over double boiler or sit bowl over pan of simmering water to create a hot water bath. In separate saucepan, warm cream. Add coconut oil and powdered sugar and whisk until smooth. Whisk warm cream mixture into melted chocolate.

Sit clean wire rack over sheet of parchment or wax paper. With fork, dip each frozen cake square in white chocolate coating. (See recipe notes.) Place on prepared rack. Decorate with edible flowers, sprinkles, sliced almonds or sugar flowers.

Allow glaze to set, about 1 hour. Serve petit fours immediately or refrigerate up to 1 day.