Preheat the oven to 180C / 350F and line an 8 ½ x 8 ½ inch tin with greaseproof paper.
In a medium bowl or jug, mix the Guinness, cornstarch, vanilla, vinegar and oil together.
In another larger bowl, mix the rest of the ingredients, except for the chocolate chips.
Slowly stir the wet ingredients into the dry with a wooden spoon.
Stir in half the chocolate chips.
Spread the mixture into the tin and top with the rest of the chocolate chips.
Bake for 25-30 minutes until a fork or toothpick inserted into the middle comes out clean.
Make sure to allow them to cool before serving as they will be gooey! I like to refrigerate mine for a bit to firm up before slicing into 9 squares.
Serve and enjoy! Keep for up to 3 days in an air-tight container.
