01 - Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan, ensuring even coverage to prevent sticking.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until evenly distributed.
03 - In a large bowl, beat the softened butter and granulated sugar using a hand mixer or stand mixer until light and fluffy, approximately 3 minutes. Add the eggs one at a time, beating well after each addition. Mix in the lemon zest, fresh lemon juice, whole milk, and vanilla extract until fully incorporated.
04 - Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Avoid overmixing. Pour the batter into the prepared pan and smooth the top with a spatula.
05 - Bake for 28 to 32 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 15 minutes.
06 - While the cake bakes, combine the blueberries, granulated sugar, and lemon juice in a small saucepan. In a separate bowl, whisk the cornstarch and water together, then stir the slurry into the blueberry mixture. Cook over medium heat, stirring constantly, until the sauce thickens, about 3 to 4 minutes. Remove from heat and let cool slightly.
07 - Using the handle of a wooden spoon, poke holes across the surface of the cooled cake, spacing them approximately 1 inch apart. Pour the warm blueberry sauce evenly over the cake, gently guiding it into the holes with a spatula. Allow the cake to cool completely.
08 - In a chilled bowl, beat the heavy whipping cream, powdered sugar, and almond extract with a mixer until stiff peaks form. Spread the whipped cream evenly over the fully cooled cake.
09 - Sprinkle the toasted sliced almonds evenly over the whipped cream layer. Refrigerate the cake for at least 1 hour before slicing and serving to allow the flavors to meld.