01 - Preheat the oven to 350°F. Grease and flour a 9×13-inch baking pan.
02 - In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, beat the softened butter and granulated sugar together until light and fluffy. Add the eggs one at a time, beating well after each addition. Mix in the lemon zest, lemon juice, milk, and vanilla extract until fully incorporated.
04 - Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined. 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 saucepan. Cook over medium heat, stirring constantly, until the mixture 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 about 1 inch apart. Pour the 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 topping. Refrigerate the cake for at least 1 hour before slicing and serving.