In a medium saucepan, whisk together the granulated sugar and cornstarch until no lumps remain.
Slowly pour in the fresh lemon juice and whole milk while continuously whisking to create a smooth liquid.
Place the saucepan over medium heat and cook the mixture, stirring constantly with a whisk to prevent it from sticking to the bottom.
Keep stirring until the liquid begins to bubble and thickens significantly into a pudding-like texture. Remove the saucepan from the heat immediately once it has thickened.
Add the unsalted butter, lemon zest and a tiny pinch of turmeric powder for color.
Stir the mixture vigorously until the butter is completely melted and incorporated into the smooth curd.
Transfer the lemon curd into a clean glass jar and press a piece of plastic wrap directly onto the surface to prevent a skin from forming.
Allow the curd to cool at room temperature, then seal the jar and refrigerate until completely chilled and set.
