Step 1: Preheat your oven to 325°F (160°C). Grease and flour a 13x9 inch baking pan, or spray generously with a baking spray that contains flour.
Step 2: In a large mixing bowl, combine the yellow cake mix, granulated sugar, vegetable oil, eggs, lemon extract, and fresh lemon zest. Mix on low speed with an electric mixer until just combined.
Step 3: Gradually add the cold 7UP soda to the batter. Increase speed to medium and mix until the batter is smooth and well-combined, about 2-3 minutes. Be careful not to overmix.
Step 4: Pour the cake batter evenly into the prepared baking pan. Gently tap the pan on the counter a few times to release any large air bubbles.
Step 5: Bake for 35-40 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean and the cake springs back when lightly touched. The cake should be golden brown.
Step 6: While the cake is baking, prepare the lemon glaze. In a medium bowl, whisk together the powdered sugar, fresh lemon juice, and 1 tablespoon of milk until smooth. If the glaze is too thick, add the remaining tablespoon of milk, or a tiny bit more if needed, until it reaches a pourable consistency.
Step 7: As soon as the cake comes out of the oven, immediately poke holes all over the top of the warm cake using a skewer, the tines of a fork, or the handle of a wooden spoon. Make sure to poke all the way through to the bottom.
Step 8: Evenly pour the prepared lemon glaze over the warm, poked cake. The warmth of the cake will help the glaze seep into the holes, making the cake incredibly moist and flavorful.
Step 9: Allow the Easy Lemon 7UP Cake to cool completely in the pan on a wire rack before slicing and serving. This allows the glaze to set properly and the flavors to fully develop.