Mix 275 ml cold water, flour and salt with a whisk until smooth. Leave to swell for 15-20 minutes. Drain plums in a sieve, collect the juice. Wash lemon thoroughly, dab dry and peel half of the peel thinly. Squeeze 1/2 lemon.
Boil up plum juice, sugar, cinnamon stick, lemon juice and peel in a pot. Stir starch and 3-4 tbsp. cold water until smooth. Remove the cinnamon stick and lemon peel from the pot, stir in the starch and let it simmer for about 1 minute while stirring. fold in plums, put them into a bowl and let them cool down
Melt the butter. Whip cream until stiff and fold into the dough in 2 portions. Heat a heart-shaped waffle iron, brush with melted butter and pour in approx. 2 tbsp. waffle dough. Bake in 4-5 minutes until golden brown.
Serve immediately or let cool on a cake rack. Bake about 14 more waffles one after the other. Dust with icing sugar for serving. Add plum roaster (compote)