01 - Preheat the oven to 350°F. Grease a 9x13-inch baking pan and set aside.
02 - In a large bowl, combine the lemon cake mix, milk, eggs, and vegetable oil. Beat on medium speed for 2 minutes until smooth and well blended.
03 - Gently fold the blueberries into the batter using a spatula, being careful not to crush them.
04 - Pour the batter into the prepared pan and spread evenly. Bake for 30 to 35 minutes, or until a toothpick inserted in 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 - Let the baked cake cool for 10 minutes. Use the handle of a wooden spoon to poke holes evenly across the entire surface. Slowly pour the warm lemon syrup over the cake, allowing it to seep into the holes. Let the cake cool completely.
07 - In a chilled bowl, beat the heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
08 - Spread the whipped cream evenly over the cooled cake. Sprinkle toasted sliced almonds over the top and garnish with fresh blueberries and lemon zest if desired.
09 - Refrigerate the cake for at least 1 hour before slicing into squares and serving.