01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan and set aside.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
03 - In a large bowl, cream the softened butter and granulated sugar together until light and fluffy. Beat in the eggs one at a time, mixing well after each addition. Stir in the lemon zest, fresh lemon juice, and vanilla extract.
04 - Alternately add the flour mixture and whole milk to the wet ingredients, beginning and ending with the flour. Mix just until combined — do not overmix.
05 - Gently fold the fresh blueberries into the batter. Pour the batter into the prepared baking pan. 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, stirring until the sugar fully dissolves. Remove from heat and set aside.
07 - Allow the baked cake to 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 absorb into the holes.
08 - Prepare the instant lemon pudding with cold milk according to the package instructions. Spread the pudding evenly over the cake surface, pressing gently so it seeps into the poked holes.
09 - Refrigerate the cake for at least 1 hour to allow the pudding layer to set and the flavors to meld.
10 - In a skillet, melt the butter over medium heat. Add the sliced almonds and sugar, stirring frequently until the almonds turn 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 cake. Scatter reserved fresh blueberries and lemon zest on top. Dust with powdered sugar if desired. Slice into squares and serve chilled.