01 - Preheat the oven to 350°F. Grease and flour a 9x13-inch baking pan, ensuring even coverage on all surfaces.
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 together until the mixture becomes light and fluffy. 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, stirring gently until just combined. Avoid overmixing. Pour the batter into the prepared baking 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 small bowl, mix the cornstarch and water to form a slurry, then stir it into the blueberry mixture. Cook over medium heat, stirring constantly, until the sauce thickens, approximately 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 about 1 inch apart. Pour the warm blueberry sauce evenly over the cake, gently nudging it into the holes. Allow the cake to cool completely.
08 - In a chilled bowl, beat the heavy whipping cream, powdered sugar, and almond extract together 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.
10 - Refrigerate the cake for at least 1 hour before serving to allow the flavors to meld and the layers to set. Slice into portions and serve chilled.