Remove the corn from the cobs by running a sharp knife down each cob. Reserve ¼ cup of corn for garnish. Chop the remaining corn finely.

Place the milk in a large saucepan and bring to a simmer (do not boil). Reduce heat to low and add the corn and onion. Cook over low heat for 15 minutes.

Pour the milk mixture into a colander set over a large bowl. Strain out the corn and onions. Set milk mixture aside.

Melt 4 tablespoons of butter in the same saucepan over medium heat. Add flour and let cook for 1 minute. Pour the milk mixture over the top, add salt and pepper, and whisk occasionally until thickened, about 5 minutes.

Melt the remaining 2 tablespoons of butter in the microwave. Pour it into the now-empty large bowl, and whisk together with the egg yolks. Pour the hot soup over the top and stir to combine.