Preheat oven to 180C (355F) and grease a 20cm x 30cm (9 x 13) baking pan and line with baking paper. Set aside. Chop up the butter in cubes and place in a medium sized pot (cutting the butter helps the butter to melt evenly and quickly). Melt butter over a low-to medium heat and mix in brown sugar and white sugar until well blended. Set aside to cool slightly (we need to add the eggs soon and we don’t want them to cook).

In a medium sized bowl, sift together the flour, baking soda and salt and then whisk them together to ensure that they are evenly distributed. Set this aside.

To the butter mixture, add vanilla, egg and egg yolk and beat until light and creamy.

Now add the sifted ingredients until just blended. Check the temperature of the dough with your hand. If it’s still really warm, it may melt the chocolate. In which case, leave in the fridge for a a few minutes to cool slightly. Once it’s cool enough, stir in the chocolate.

Spoon batter into the prepared baking pan and bake for 20-30 minutes or until golden brown. When cool, lift the entire slice out of the pan using the sides of the baking paper and cut into squares.