Pour 200 ml (¾ cup) of hot water into a jug or large cup and add three teabags. Let them steep for 5 minutes, then remove the teabags.
Measure out 150 ml (⅗ cup) of the tea and add the organic dried mulberries and raisins. Stir to coat the fruit evenly with the tea. Let the tea and fruit mixture cool, then cover and refrigerate overnight (or for at least 6–8 hours) until the fruit is plump and most of the liquid has been absorbed.
When you’re ready to make the cake, take the soaked fruit out of the fridge and let it sit for 10–15 minutes to warm slightly.
Preheat the oven to 175°C (350°F).
Grease and line a small loaf tin—about 18 × 10 × 7 cm (7 × 4 × 3 in) / 500 ml capacity (see note 4).
In a large bowl, whisk together the egg, brown sugar, and date syrup.
Stir in the soaked fruit mixture (including any remaining liquid), orange zest, and a pinch of salt.
Sift the spelt flour and baking powder together, then gently fold them into the wet ingredients until just combined. The batter should be thick but spoonable. If it feels too stiff, add 1 tsp of the brewed tea. If too loose, add 1 tsp flour.
Spoon the batter into the prepared tin (it should almost reach the top).
Bake on the lower rack for 40–45 minutes (check at 30 minutes).
A skewer should come out clean or with a few moist crumbs, but not sticky batter. If the top browns too quickly, loosely cover it with foil near the end.
Leave the loaf to cool in the tin for 10 minutes, then lift it out onto a wire rack. Cool completely before glazing.