01 - Preheat oven to 350°F. Grease and flour a 9x13-inch baking pan.
02 - In a large mixing bowl, combine the lemon cake mix, milk, eggs, and vegetable oil. Beat on medium speed for 2 minutes until the batter is smooth and free of lumps.
03 - Gently fold the blueberries into the batter using a spatula, being careful not to crush them or overmix.
04 - Pour the batter into the prepared pan and spread evenly. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
05 - While the cake bakes, combine the lemon juice and granulated sugar in a small saucepan. Heat over medium-low, stirring constantly until the sugar fully dissolves. Remove from heat and set aside.
06 - Remove the cake from the oven and allow it to cool for 10 minutes. Using the handle of a wooden spoon, poke holes evenly across the entire surface of the cake. Slowly pour the warm lemon syrup over the cake, allowing it to seep into the holes.
07 - Let the cake cool completely to room temperature in the pan.
08 - In a chilled mixing bowl, beat the heavy cream, powdered sugar, and vanilla extract with an electric mixer until stiff peaks form.
09 - Spread the whipped cream evenly over the cooled cake. Sprinkle the toasted sliced almonds over the top and garnish with additional blueberries and lemon zest if desired.
10 - Refrigerate the cake for at least 1 hour before slicing into squares and serving.