01 - Preheat the oven to 350°F. Grease and flour a 9x13-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, whole milk, and vanilla extract until fully combined.
04 - Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined. Do not overmix. Pour the batter into the prepared baking pan and smooth the top evenly.
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, mix the cornstarch and water until smooth, then stir into the saucepan. 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 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 - 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.