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 until evenly incorporated.
04 - Alternate adding the flour mixture and whole milk to the wet ingredients, beginning and ending with the flour mixture. Mix just until combined—do not overmix.
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 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 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 seep into the holes.
08 - Prepare the instant lemon pudding with cold milk according to the package instructions. Pour or spread the pudding evenly over the cake, pressing gently so it fills the poked holes.
09 - Place the cake in the refrigerator and chill for at least 1 hour to allow the flavors to meld and the pudding to set.
10 - In a skillet, melt the butter over medium heat. Add the sliced almonds and sugar, stirring frequently until the almonds are golden and fragrant, about 3 to 4 minutes. Remove from heat and let cool completely.
11 - Before serving, sprinkle the toasted almond topping over the chilled cake. Scatter reserved fresh blueberries and lemon zest on top. Dust with powdered sugar if desired.