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 fully 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. Gently fold in the fresh blueberries.
05 - Pour the batter into the prepared 9x13-inch 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, stirring occasionally, until the sugar fully dissolves. Remove from heat and set aside.
07 - Let the baked cake 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 or spread the pudding evenly over the cake, pressing gently so it seeps into 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 over medium heat, melt the butter. 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.