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 overmix.
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 absorb into the holes.
07 - Allow the cake to cool completely at room temperature before adding the topping.
08 - In a chilled bowl, beat the heavy cream, powdered sugar, and vanilla extract with a mixer until stiff peaks form.
09 - Spread the whipped cream evenly over the cooled cake. Sprinkle the toasted sliced almonds on top. Garnish with additional blueberries and lemon zest if desired.
10 - Refrigerate the cake for at least 1 hour before slicing into squares and serving.