01 - Preheat oven to 350°F. Grease and flour a 9x13-inch baking pan.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, 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, fresh lemon juice, and vanilla extract.
04 - Gradually add the flour mixture to the wet ingredients, alternating with the whole milk. Begin and end with the flour mixture, mixing just until combined after each addition.
05 - Gently fold the fresh blueberries into the batter. Pour the batter into the prepared baking pan and spread evenly. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
06 - While the cake bakes, combine the fresh lemon juice and granulated sugar in a small saucepan. Heat over low heat, stirring constantly, until the sugar has completely dissolved. Remove from heat and set aside.
07 - Remove the cake from the oven and let it cool for 10 minutes. Use the handle of a wooden spoon to poke holes across the entire surface of the cake, spacing them about 1 inch apart. Slowly pour the warm lemon syrup evenly over the cake, allowing it to soak into the holes.
08 - Prepare the instant lemon pudding with cold milk according to the package instructions. Pour the pudding evenly over the cake, gently spreading it and pressing down so the pudding seeps into the poked holes.
09 - Place the cake in the refrigerator and chill for at least 1 hour to allow the pudding to set and the flavors to meld.
10 - In a skillet over medium heat, melt the butter. Add the sliced almonds and sugar, stirring frequently, until the almonds are golden brown and fragrant, about 3 to 4 minutes. Transfer to a plate and let cool completely.
11 - Before serving, sprinkle the toasted almond topping over the chilled cake. Scatter fresh blueberries and lemon zest on top, and dust with powdered sugar if desired.