Lightly coat the inside of a 5- to 6-quart slow cooker with nonstick cooking spray or a thin smear of butter, and line it with parchment paper if desired.

Pour the dry yellow cake mix into a medium bowl and stir in 1/2 cup of the melted butter until no dry patches remain and a thick, slightly crumbly dough forms.

Transfer the cake mixture to the prepared slow cooker and press it firmly into an even layer across the bottom.

In a large bowl, beat the softened cream cheese with a hand mixer on medium speed for 1 to 2 minutes, until smooth and creamy.

Add the remaining 1/2 cup melted butter and beat until fully combined, then add the powdered sugar about 1 cup at a time on low speed before increasing to medium and beating until thick, glossy, and smooth.

Spread the cream cheese mixture over the cake base all the way to the edges.

Cover the slow cooker and cook on LOW for 2 1/2 to 3 1/2 hours, checking around 2 1/2 hours, until the edges are set and lightly golden, the top is puffed and slightly crinkly, and the center still jiggles softly.

Turn off the slow cooker and let the cake sit covered for 15 to 20 minutes so it can set slightly while remaining gooey and creamy in the center.

Scoop the warm cake into small bowls or onto dessert plates, or cool it longer and cut it into squares for cleaner slices.