Preheat the oven to 180°C/350°F/Gas Mark 4. Spray the mini bundt trays with cake release oil.
Put the caster sugar, eggs, butter and oil into a large bowl and whisk together well for 5 minutes, until pale and fluffy. Add the lemon zest and yogurt and mix again.
Sift in the flour, baking powder and salt, and gently fold in, using a metal spoon, until all the flour is incorporated. Carefully pour the mixture into the mini bundt trays.
Bake for 20–30 minutes or until the cakes are a light golden colour and a cocktail stick inserted into the centre comes out clean. Leave the babkas to cool in the trays. Turn out when cool enough to handle.
When cool, lightly dust with the powdered icing sugar.