Preheat the oven to 325 F/ 170 C/ fan 150 C/ gas mark 3. Grease your cake pan thoroughly (with butter or margarine) and coat with almond flour or breadcrumbs. Turn the pan upside down to remove excess crumbs and set aside.
Combine the flour with the potato starch, baking powder and salt and stir thoroughly using a whisk. Set aside.
Beat together the sugar and eggs (on med/high, speed 6) for 5-8 minutes or until pale, thick and fluffy.
Slowly pour in the oil and beat (on low) until thoroughly incorporated. Add the vanilla extract.
Gradually add the dry ingredients using a whisk until just combined.
Pour approx. ⅔ of the batter into the prepared pan.
Stir the cocoa powder along with 1½ tablespoons of water into the remaining batter until thoroughly incorporated.
Pour the cocoa mixture into the cake pan (try to distribute it evenly over the surface of the cake). Some of this mixture may sink into the batter.
Bake in the centre of the oven for 1 hour or until the skewer inserted into the middle comes out clean.
Remove from the oven and set aside to cool. Remove from the pan while still a little warm (gently tap the pan against the counter a couple of times before turning over). Cool the babka completely before dusting with powdered sugar/icing sugar and serve.
Notes
I recommend weighing the ingredients rather than using measuring cups, if possible.
You can buy potato starch in Polish shops (look for 'mąka ziemniaczana').
Make sure your eggs are at room temperature.
It’s important to beat the eggs and sugar until pale and thick so take your time doing this step.
I recommend greasing the pan with either butter or margarine rather than oil. The ground almonds will stick to the pan better. Grease your pan even if it is non-stick.
Cool your babka cake completely before decorating and cutting. This cake tastes better the next day so it's well suited to making ahead. Perfect with coffee or tea!
Keep covered with foil/cling film or in an airtight container (to ensure it retains its moisture) in the fridge for up to 5 days.
Freeze for up to 3 months. Decorate after defrosting.