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, beat the softened butter and granulated sugar together until pale, light, and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the lemon zest, fresh lemon juice, whole milk, and vanilla extract until fully incorporated.
04 - Gradually fold the dry ingredients into the wet mixture, stirring until just combined. Pour the batter into the prepared baking pan and smooth the top with a spatula. Bake for 28–32 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 15 minutes.
05 - 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 saucepan. Cook over medium heat, stirring constantly, until the mixture thickens, approximately 3–4 minutes. Remove from heat and let cool slightly.
06 - 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.
07 - 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.
08 - Sprinkle the toasted sliced almonds evenly over the whipped cream topping. Refrigerate the cake for at least 1 hour before slicing and serving.