Clean and core the quinces. Quarter them, then cut each quarter into three slices. Add Kambucha or apple cider, honey, vanilla, lemon zest, lemon juice, and sugar to a saucepan. Place over low heat, bring to a boil, then cook for 30 - 35 minutes. Strain the mixture through a fine sieve into a bowl. Keep both the syrup and quinces for later. Leave it to cool completely, then use.
Honey Hazelnut Quince Cake
Preheat your oven to 170 °C or 340 °F. Grease a loaf pan (12 x 24 cm or 5x9 inches) with butter, then line with parchment paper, leaving a short overhang on each long side (this way it will be easier to transfer the baked cake). Make the cake batter. In a bowl cream the butter, sugar, vanilla, and honey. Mix on high speed for about 2 - 3 minutes. Gradually, while mixing, add the eggs to the butter mixture, one by one. Only add the next egg once the previous one has been completely incorporated. In a separate bowl, stir to combine whole wheat flour, all-purpose flour, baking powder, a pinch of salt, and ground hazelnuts. Alternate adding dry ingredients (flour mixture) and 100g / 3.5 oz of prepared cooled quince syrup to the butter mixture until well incorporated and smooth. Mix on low speed until you get a smooth batter.
Bake and Serve
Pour the cake batter into the prepared loaf pan. Arrange the poached quinces on top. Place in the preheated oven, on the middle rack. Bake for 50 - 60 minutes at 170 °C or 340 °F or until a toothpick inserted in the middle comes out clean. Take the baked Honey Hazelnut Quince Cake from the oven, leave the cake in the pan for 10 minutes, then brush with the rest of the quince syrup and transfer to a wire rack to cool. You can serve this cake while still warm or at room temperature. Cut on slices and enjoy.