Preheat the oven to 180°C/160℃ fan/350°F/gas mark 4. Line a deep 20 cm (8 in) or a 23 cm (9 in) square cake tin with baking paper.
To make the streusel topping, mix the flour, sugar and cinnamon together in a bowl. Add the butter and rub it in using your fingertips until no lumps of butter remain. Place in the fridge.
To make the cinnamon sugar layer, mix the sugar and cinnamon together in a bowl. Set aside.
To make the cake batter, whisk together the yogurt, milk, oil, vinegar, caster sugar and vanilla extract in a large bowl until very well combined.
Whisk in the cornflour, baking powder and soda and salt then add the plain flour and gently stir with a balloon whisk until no dry lumps remain.
Pour half of the batter into the lined tin and spread it level. Sprinkle over the cinnamon sugar evenly.
Dollop over the rest of the batter and gently spread it out with a spatula or the back of a spoon. It will not want to stick to the cinnamon sugar so just gently coax it and try to lift the spoon/spatula as little as possible.
Scatter over the streusel topping evenly.
Bake for 35-45 minutes until firm to the touch and a skewer inserted into the centre comes out clean (apart from a bit of cinnamon).
Leave the cake to cool in the tin before slicing.