Preheat the oven to 350°F (175°C). Prepare the lemon cake batter according to the package instructions.
Pour the batter into a greased 9×9-inch square baking pan and bake for 28-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool completely in the pan on a wire rack.
While the cake cools, prepare the whipped cream by combining the heavy whipping cream, granulated sugar, and vanilla extract in a large mixing bowl.
Using an electric mixer, beat the cream mixture on medium-high speed until stiff peaks form. Set aside.
Once the cake is cooled, cut it into 1-inch cubes.
In a large trifle bowl or individual serving glasses, layer one-third of the lemon cake cubes evenly on the bottom.
Spread half of the lemon curd evenly over the cake layer.
Add one-third of the whipped cream over the lemon curd layer.
Scatter one-third of the blueberries and raspberries over the whipped cream.
Repeat the layers: lemon cake cubes, remaining lemon curd, whipped cream, and berries.
Finish with a final layer of lemon cake cubes and a generous layer of whipped cream on top.
Garnish with remaining berries and dust with powdered sugar if desired.
Refrigerate the trifle for at least 2 hours before serving to allow the flavors to meld.