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 using an electric mixer until pale, 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 until evenly incorporated.
04 - Alternately add the flour mixture and whole milk to the wet ingredients in three additions, beginning and ending with the flour. Stir gently just until combined — do not overmix.
05 - Gently fold the fresh blueberries into the batter. Pour the batter into the prepared pan and spread evenly. Bake for 30 to 35 minutes, or until a toothpick inserted in 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, stirring constantly, until the sugar fully dissolves. Remove from heat and set aside.
07 - Allow the baked cake to cool in the pan for 10 minutes. Using the handle of a wooden spoon, poke holes across the entire surface of the cake, spacing them about 1 inch apart. Slowly and evenly pour the warm lemon syrup over the cake, allowing it to seep into the holes.
08 - Prepare the instant lemon pudding with cold milk according to the package directions. Spread the pudding evenly over the warm cake, gently pressing it into the poked holes. Refrigerate the cake for at least 1 hour to set.
09 - 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.
10 - Before serving, scatter the toasted almond topping, reserved fresh blueberries, and additional lemon zest over the chilled cake. Dust with powdered sugar if desired.