Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish.

In a large bowl, combine the yellow cake mix.

In another bowl, whisk together the melted butter, eggs, and milk. Pour this mixture into the cake mix, stirring gently until just blended.

In a separate bowl, mix the softened cream cheese with sugar until smooth. Add the fresh lemon juice and lemon zest, blending well.

Pour the cake batter into your prepared dish, then dollop the cream cheese mixture on top.

If you're making the blueberry variation, sprinkle the blueberries over the cake batter before adding the cream cheese mixture.

Using a knife, gently swirl the cream cheese mixture into the batter.

Place the dish in the oven and bake for 30-45 minutes, or until the top is golden brown and a toothpick comes out clean.

Let your cake cool before glazing with a simple mixture of powdered sugar and lemon zest.