01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan with butter or nonstick spray.
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 using an electric mixer. 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 - Alternate adding 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 and spread evenly. Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean.
06 - While the cake bakes, combine the fresh lemon juice and sugar in a small saucepan. Heat over low, stirring occasionally, until the sugar fully dissolves. Remove from heat and set aside.
07 - Remove the cake from the oven and let it cool 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 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 over the cake and spread evenly, gently pressing so it 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 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 3–4 minutes. Remove from heat and let cool completely.
11 - Before serving, sprinkle the toasted almond topping, reserved fresh blueberries, and additional lemon zest over the cake. Dust with powdered sugar if desired.