Add 2 cups heavy cream, sugar, honey and vanilla extract to a saucepan. Heat until sugar is dissolved, about 2-3 minutes. Add 1/4 cup of cream mixture (you can eyeball this measurement) to the jar of a blender, along with the roasted plums and their juices. Blend until very smooth, about 1 minute. Place a sieve atop a large bowl and pour the puree through it, discarding the pulp. Add the remaining milk to the bowl and mix until thoroughly combined. Cover the bowl with plastic wrap and refrigerate until very cold, at least 4 hours of overnight.