Prepare the boxed cake mix according to the package instructions and bake as directed.
Let the cake cool completely, then remove it from the pan by placing a cutting board over the pan and flipping it over. The cake should slide right out.
Make the Lemon Simple Syrup: In a small saucepan, combine granulated sugar, water, and ¼ cup lemon juice over medium-high heat.
Whisk until the sugar dissolves, then simmer until the mixture reduces by half and thickens into a syrup.
Remove from heat and let cool.
Prepare the Whipped Cream: Let the cream cheese soften slightly on the counter for about 5 minutes (it should still be cold).
In a mixing bowl, whip the cream cheese on medium speed until fluffy.
Add powdered sugar and beat until combined.
Slowly pour in heavy cream while mixing on medium speed for 3-4 minutes.
Add lemon zest and the remaining ¼ cup lemon juice, then continue mixing until stiff peaks form (about 5 minutes). Be careful not to overmix.
Refrigerate the whipped cream until ready to use.
Assemble the Bars: Place the cake on a serving platter or keep it on the cutting board.
Slice the cake horizontally into two even layers. (Tip: Freezing the cake for 5 minutes before cutting makes it easier.)
Set the top half aside and brush both layers with lemon simple syrup using a pastry brush.
Filling the Cake: Spread half of the Red Raspberry Fruit Filling evenly over the bottom cake layer (about ¼” thick).
Spread a thin layer (¼”–½”) of whipped cream on top of the raspberry filling.
Place the second cake layer on top.
Spread the remaining raspberry filling over the second layer.
Top with a thick layer of whipped cream.
Refrigerate until ready to serve to allow the cake to set.
Cut into 12-16 bars.
Garnish with fresh raspberries, lemon zest, or lemon slices. Enjoy!
