Heat the oven to 175 C / 350 F. Grease a 9x13-inch baking pan with butter or nonstick spray, then line with parchment paper, leaving an overhang on two sides for easy lifting.

Combine the dry yellow cake mix, melted butter, and 1 egg in a large mixing bowl. Stir with a rubber spatula until a thick, crumbly dough forms.

Transfer the dough to the prepared pan and press it firmly into an even layer across the bottom using the flat bottom of a measuring cup. Press into the corners so there are no thin spots.

Beat the softened cream cheese with a hand mixer on medium speed for about 2 minutes until completely smooth with no lumps.

Add both eggs and the vanilla extract and beat on low until just combined, about 30 seconds.

Add the sifted powdered sugar in two additions, mixing on low after each until the filling is smooth and pourable. Scrape down the bowl sides once.

Pour the cream cheese filling evenly over the pressed crust layer. Tilt the pan gently to spread it into the corners if needed.

Bake for 38 to 42 minutes, until the edges are golden brown and the center jiggles as one cohesive mass rather than rippling like liquid.

Remove from the oven and cool in the pan on a wire rack for 30 minutes at room temperature.

Transfer the pan to the refrigerator and chill for at least 2 hours, or overnight, until the filling is fully set.

Lift the slab out using the parchment overhang. Place on a cutting board and cut into 24 squares using a sharp knife, wiping the blade clean between cuts for neat edges.

Dust with powdered sugar just before serving if desired.