Step 1

Preheat the oven to 180C/160C fan forced. Brush a straight-sided 20cm square cake pan with extra melted butter to grease.

Step 2

Place 215g (1 cup) sugar and 60ml (1/4 cup) water in a heavy-based saucepan over medium-high heat. Stir constantly until the sugar dissolves. Simmer, without stirring, brushing down the side of pan with a wet pastry brush to prevent sugar crystals forming, for 8 minutes or until just golden. Remove from heat and set aside until the bubbles subside. Pour into the prepared pan and tilt to evenly cover the base. Place the pan in a large roasting pan.

Step 3

Sift the flour, cocoa, baking powder and bicarb into a bowl. Use electric beaters to beat the butter and remaining sugar in a large bowl until pale and creamy. Beat in 1 egg and half the vanilla until just combined. Beat in half the flour mixture and half the buttermilk until just combined. Repeat with the remaining flour mixture and buttermilk. Spread the cake mixture evenly over the caramel layer.

Step 4

Whisk together the condensed milk, evaporated milk and remaining eggs and vanilla in a large jug or bowl. Pour over the back of a wooden spoon over the cake mixture. Cover the cake pan with foil. Pour enough boiling water into the roasting pan to come halfway up the sides of the cake pan.

Step 5

Bake for 1 hour or until the chocolate cake comes away from the sides of the pan. Remove cake pan from the roasting pan and set aside for 2 hours to cool completely. Carefully run a flat-bladed palette knife around the sides of the pan. Turn the slice onto a serving platter with a lip (to catch the dripping caramel sauce). Cut into slices to serve.