Place 1/2 cup butter in an 8x8 or 9x9 inch glass pan. Melt it in the microwave directly in the pan. (For a metal pan, melt butter in a bowl and then transfer it to the pan.) Using a spatula or pastry brush, coat the pan's sides with butter to avoid sticking.
Drain the can of whole kernel corn. Add the drained corn into the pan with the butter.
Empty the can of creamed corn entirely into the pan.
Stir in 1 cup sour cream.
Mix in 3 tablespoons sugar and 1/2 teaspoon kosher salt (if using table salt, use 1/4 teaspoon).
Add the 8-ounce box of Jiffy corn muffin mix.
Stir all ingredients with a spatula until thoroughly combined. Scrape down the pan's edges.
Bake at 350°F (175°C) for about 35-40 minutes. The edges should turn golden brown and start to pull away from the pan. When the pan is shaken, the center should jiggle no more than the edges. Alternatively, check with a toothpick: if it comes out clean with no wet batter, it's ready.
Allow to cool on a wire rack for a couple minutes, then serve warm.