Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Coarsely chop the hazelnuts. Put flour, butter in pieces, 2 tablespoons of sugar, baking powder, 1 pinch of salt and chopped hazelnuts in a bowl. First work into crumbles with the dough hooks of the mixer, then with your hands. Spread the crumbles on a baking tray lined with baking paper and bake for 10-12 minutes. Leave to cool.
Wash, clean and quarter the strawberries. Mix half the strawberries with 2 tablespoons of sugar and liqueur, set aside. Finely puree the remaining strawberries in a tall mixing bowl.
Whip the cream until stiff. Mix the sour cream and 2 tablespoons of sugar. Fold cream into the sour cream. Fold in the strawberry puree. Arrange cream and strawberries together with marinade on plates. Sprinkle with crumbles. Dust with icing sugar. Decorate with lemon balm as desired.