Preheat oven to 300 degrees. Pour tomatoes into a 13x9 pan and cut in half lengthwise. Add the garlic, cayenne pepper, and half the basil. Drizzle with 1/4 cup olive oil and sprinkle with salt and pepper. Roast in the oven for 90 minutes. Discard cooked basil.

Transfer tomato mixture to a blender and blend well. Heat the milk in a saucepan on the stove on low until it starts to foam but doesn't boil. Stream hot milk into the tomato mixture while blending. Season to taste with salt and pepper. Garnish with remaining basil.