First, wash and dry the grapes. Pull grapes off the stems and set aside.
Then sprinkle the gelatin onto 50ml of water and let it sit to bloom.
Whip the cream to stiff peaks.
Cut the castella into 1 cm thick slices.
Combine grape juice and sugar in a saucepan and heat until sugar has dissolved. Stir in gelatin and mix to dissolve completely.
Put some ice in a large bowl and place a smaller bowl inside. Pour mixture into the smaller bowl and cool until slightly thickened.
Fold in the yogurt and whipped cream and gently mix until incorporated.
Pour mixture over the castella slices and smooth out the surface. Arrange the grapes on top, making sure to push the grapes halfway into the yogurt mixture.
Refrigerate until firm.
Slice into individual portions and brush on honey if desired.
Original article and pictures take https://www.tastemade.com/videos/grape-mousse-cake site