Onion Preparation: Initiate by preheating the oven to 350˚F (175˚C). Over a low flame, sauté the segmented Vidalia onions in 1/2 cup of the unsalted butter for an approximate duration of 10 minutes or until they achieve a translucent appearance. Subsequently, integrate the poppy seeds, salt, paprika, and ground pepper into the sautéed onions. Combine these ingredients thoroughly and then remove the pan from the heat source.

In a sizable mixing bowl, amalgamate the all-purpose flour, cornstarch, baking powder, and brown sugar. Integrate 1 1/4 cups of the portioned butter into the dry ingredients. This should result in a dough of a fragmented and granular consistency.

In a separate vessel, proceed to melt the remaining butter. Concurrently, in another bowl, amalgamate the eggs, whole milk, sour cream, and the previously melted butter, ensuring a homogenous mixture.

Construct a well in the center of your dry mixture. Subsequently, transfer the liquid ingredients into this well. Mix until the wet and dry components are uniformly combined, ensuring not to overmix.

Transition the batter into a lubricated 10-inch springform pan. Delicately layer the previously prepared caramelized onions atop the batter. Commit the pan to the pre-heated oven and allow baking for a time interval ranging between 35 to 45 minutes. A completed bake can be verified by inserting a knife into the center; it should emerge devoid of batter remnants. Post-baking, permit the cake to cool within its pan for a minimum duration of 5 minutes before the subsequent extraction.