Place corn kernels, corn cobs, cream, milk and 1/2 cup sugar in a saucepan and bring to a simmer over medium heat, stirring occasionally. Remove from heat, cover and let steep for 1 hour.

Remove cobs and puree mixture in a blender. Strain through a mesh strainer, pressing on solids with a wooden spoon, and return to saucepan. Bring to a boil, taking care not to scald mixture, and remove from heat.

Combine egg yolks, salt and remaining 1/2 cup sugar in a bowl and whisk until mixture is thick and pale yellow.

Slowly add 1 cup of hot cream mixture to egg mixture, whisking constantly, then add egg mixture to saucepan and simmer over low heat, stirring often, until mixture reaches 160-170 degrees. Add vanilla extract and stir to incorporate.

Chill custard in refrigerator or an ice bath, then freeze in an ice cream mixture. When frozen, transfer ice cream to a quart container, adding dollops of blackberry jam intermittently throughout and stirring gently to create swirls.