01 - Preheat the oven to 350°F. Grease a 9x13-inch baking pan with butter or nonstick spray.
02 - In a large mixing bowl, combine the lemon cake mix, milk, eggs, and vegetable oil. Beat on medium speed with a hand mixer or stand mixer 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 overmix or crush the berries.
04 - Pour the batter into the prepared pan and spread it 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 let it cool for 10 minutes. Use the handle of a wooden spoon to poke holes evenly across the entire surface of the cake.
07 - Slowly pour the warm lemon syrup over the cake, allowing it to seep into the holes. Let the cake cool completely to room temperature.
08 - In a chilled mixing bowl, beat the heavy cream, powdered sugar, and vanilla extract on medium-high speed until stiff peaks form.
09 - Spread the whipped cream evenly over the cooled cake. Sprinkle the toasted sliced almonds over the 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.