Preheat the oven to 175 Degrees C and line a 9 inch loaf tin by brushing it with oil and baking paper, making sure it hangs over the sides so it is easier to lift from the tin once baked.

In a smaller mixing bowl add the cocoa powder, espresso powder and hot water - stir together until smooth and combined. Set aside.

In a large mixing bowl add the flour, baking powder, baking soda, salt and stir together with your fingers. Add the butter and using your finger rub together to create a damp sand like texture. Add in the castor sugar and stir together.

Add the eggs, vanilla, yogurt and whisk until smooth.

Divide the batter into half and using a rubber spatula transfer half the batter to the bowl with the cocoa powder mixture. Whisk it well until smooth and combined.

Scoop 3-4 dollops of vanilla batter at the bottom of the tin and fill in the gaps with the chocolate batter. Top the vanilla batter with scoops of chocolate. Then top the chocolate batter with scoops of vanilla batter and repeat unit the batters are done.

Using a chopstick or a knife, swirl the batters together a couple of times and bake in the oven for 45-50 minutes or until a tooth pick inserted into the center comes out clean.

While the cake is baking make the simple syrup by boiling the water and sugar together until the sugar has dissolved and the syrup is transparent.

Once the cake is out of the oven, poke holes into it and pour over the warm syrup. Allow this to soak for a couple of hours covered with cling wrap.

To make the cocoa glaze, stir all the ingredients together until smooth and pour over the cake, sprinkle over the sea salt flakes, slice and serve.