Preheat the oven to 180°C/160℃ fan/350°F/gas mark 4. Line a deep 23 cm (9 in) square cake tin with baking paper, making sure that you leave some overhanging on either side so that you can easily lift the cake out later.
To make the streusel topping, mix together the melted butter, sugar, cinnamon and salt in a bowl then stir in the flour, make sure there are no dry patches. Place the bowl in the fridge while you make the cake batter.
115 g (½ cup) vegan block butter, 120 g (⅔ cup) light brown soft sugar, 1 teaspoon cinnamon, ¼ teaspoon salt, 210 g (1 + ¾ cups) plain (all-purpose) flour
To make the cake, whisk together the melted butter, treacle, golden syrup, yogurt, milk and sugar in a large bowl.
115 g (½ cup) vegan block butter, 170 g (½ cup) black treacle (or molasses), 85 g (¼ cup) golden syrup, 160 g (⅔ cup) plain non-dairy yogurt (ideally Greek style), 120 g (½ cup) unsweetened non-dairy milk (I use soy), 50 g (¼ cup) light brown soft sugar
Whisk in the baking powder, bicarbonate of soda, salt, ginger, cinnamon, cloves and nutmeg.
¾ teaspoon baking powder, ¾ teaspoon bicarbonate of soda (baking soda), ½ teaspoon salt, 2 ½ teaspoons ground ginger, 2 teaspoons cinnamon, ½ teaspoon ground cloves, ½ teaspoon ground nutmeg
Finally add the flour and stir with a balloon whisk until no dry lumps remain.
260 g (2 cups + 2 ½ Tablespoons) plain (all-purpose) flour
Scrape the cake batter into the lined cake tin and spread it level.
Use your hands to break the crumb topping into small pieces of varying sizes and scatter them evenly over the cake batter.
Bake for about 35-45 minutes until firm to the touch and a skewer inserted into the centre comes out clean.
Leave the cake to cool in the tin completely before slicing into squares.