Step 2

Place the potatoes in a medium saucepan, cover with about 500ml of cold water and bring to a boil. Reduce heat to medium-low and simmer until the potatoes are soft (about 15 minutes). Drain the potatoes and reserve the cooking water.

Step 4

Mash the potatoes, then transfer into the large bowl of a cake mixer. Add 125ml of the reserved potato water (it will still be warm), 90g of the diced butter and the caster sugar. With the paddle attachment in place, blend on low speed until the butter has melted and the mix is smooth. Set aside.

Step 6

Place the yeast in a small bowl and add 60ml of the lukewarm potato cooking-water. Stir and let stand for 5 minutes, then add to the mashed potato mixture, followed by the egg yolks. Mix on low to combine, then add the flour and salt. Change to the dough hook attachment and mix on low speed until it forms a rough ball (about 5 minutes), then scrape the dough out onto a lightly floured worktop and knead gently for about 5 minutes until the dough is smooth, soft and supple.

Step 8

Grease a medium bowl with 1 tablespoon of the softened butter and roll the dough around in the bowl. Cover the bowl with a tea towel and place a plate on top, then put the bowl in the fridge to allow the dough to rise overnight. (If making the cake on the same day, place the covered bowl in a warm place for 1-1½ hours, then proceed with the recipe once the dough has doubled in size.)

Step 10

The next morning, grease the base and sides of a baking tray measuring about 25cm x 35cm x 4cm with the remaining 1 tablespoon of softened butter. Remove the dough from the fridge – it should be nearly double its original size – and punch it down, then transfer to the greased baking tray. Press and stretch the dough to fit the base of the tray, then cover with a towel. Leave in a warm place for about an hour (or 30 minutes if the dough was not refrigerated) until puffy.

Step 12

Preheat the oven to 180C fan-forced (200C conventional). Using your fingers, press directly down into the puffed-up dough to create deep dimples all over, then scatter the remaining 80g of diced butter on top. Combine the brown sugar, cinnamon and walnuts (or pecans) in a bowl, then sprinkle this mix over the buttered dough. Drizzle the cream on top, then bake for about 30 minutes, until the cake is browned and bubbling. Transfer to a cake rack to cool for about 10 minutes before cutting into large squares to serve.

Step 13

Tip: This is best eaten warm; just reheat leftovers in the oven.