Heat your oven to 350°F and generously grease a 9x13 inch baking dish (or similar size) with butter or cooking spray, making sure to coat the bottom and all sides evenly. This prevents sticking and helps the cake bake evenly. Have your oven rack positioned in the center, as this ensures the cake bakes uniformly without browning too quickly on top.

While the oven preheats, toss the blueberries with 1 teaspoon of flour in a small bowl—this prevents them from sinking to the bottom during baking. In a large mixing bowl, whisk together the lemon cake mix and instant lemon pudding mix, breaking up any lumps. I like to sift these dry ingredients together to ensure there are no clumps that won't incorporate smoothly into the batter.

Add the room temperature eggs, water, vegetable oil, lemon juice, lemon zest, and sour cream to the dry mixture from Step 2. Using an electric mixer on medium speed, beat for about 2 minutes until the batter is smooth and well combined—this develops the structure of the cake and ensures even leavening. The room temperature eggs emulsify better with the oil, creating a more tender crumb. Fold in the flour-coated blueberries from Step 2 gently at the end, using a spatula to avoid crushing them.

Pour the batter into the prepared pan from Step 1 and smooth the top with a spatula. Bake in the preheated 350°F oven for 40 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs (not wet batter, but not completely dry either). The cake will be golden brown and spring back slightly when lightly touched.

While the cake bakes (around the 35-minute mark), prepare the glaze by whisking together the sifted powdered sugar, water, and vanilla extract in a small bowl until smooth and pourable. Once the cake comes out of the oven, let it cool in the pan for 10 to 15 minutes—this allows it to set slightly so the glaze absorbs without running off completely. Then turn the cake out onto a wire cooling rack.

Once the cake has cooled to lukewarm (about 10-15 minutes on the rack), pour the glaze from Step 5 over the top, allowing it to drip down the sides naturally. I find that pouring the glaze when the cake is still warm helps it penetrate slightly and creates a beautiful, even coating. Let the glaze set for about 10 minutes before slicing—this gives it time to firm up while still maintaining that lovely glossy finish.