01 - Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch (23x33 cm) baking pan.
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, cream the softened butter and granulated sugar until light and fluffy. Beat in the eggs one at a time, ensuring each is fully incorporated. Mix in the lemon zest, fresh lemon juice, whole milk, and vanilla extract until combined.
04 - Gradually add the dry ingredients to the wet mixture, mixing until just combined. Pour the batter into the prepared pan and smooth the top evenly.
05 - Bake for 28–32 minutes, or until a toothpick inserted in the center comes out clean. Let the cake 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, mix the cornstarch and water to form a slurry, then stir it into the saucepan. Cook over medium heat, stirring constantly, until the mixture thickens, about 3–4 minutes. Remove from heat and let cool slightly.
07 - Using the handle of a wooden spoon, poke holes across the cooled cake, spacing them about 1 inch apart. Pour the blueberry sauce over the cake, spreading it evenly and gently nudging the sauce into the holes. Allow the cake to cool completely.
08 - 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 topping. Refrigerate the cake for at least 1 hour before slicing and serving.