Using Balsamic Vinegar for the caramel that the pears are laid into, gives a deeper, darker and richer flavour with added stickiness, making it perfect to hold the pears in place. More pears in the cake batter make sure that the cake is full of fruity flavour and kept moist while cooking.
Grease and line, with parchment paper, a deep 20cm spring form cake tin and place on a baking sheet. Heat the oven to 170C.
Put the butter, 120g sugar and the Balsamic Vinegar in a pan on a medium heat. As the butter melts, stir the mix until the sugar dissolves and then bring it up to a gentle boil for 1 -2 minutes. Pour the melted mixture into the base of the cake tin, taking care as this will be extremely hot and sticky.
Remove the stalks, cut in half and core two of the pears. Cut into thin slices, length ways and arrange the pear slices into a fan pattern on top of the caramel, making sure that there are no gaps showing. Grate the flesh from the remaining pears into a bowl and set aside. Mix the flour with the baking powder and polenta and leave to one side.
Add the 3 eggs and 150g sugar into a large mixing bowl and whisk with an electric mixer for 3-4 mins until pale and fluffy. Whilst continuing to whisk pour in the olive oil and whisk for a further 2-3 mins. Use a spatula to fold the pears into the egg mix, followed by the flour mix adding a third at a time until all the flour has been combined. Scrape the cake mix into the cake tin and place in the oven and bake for 45 mins or until a skewer comes out clean.
Allow to cool in the cake tin on a wire rack for 15-20 minutes. Remove the side of the tin by releasing the spring, and place a plate on top of the cake and invert. Carefully remove the bottom of the tin and the baking paper. Serve with crème fraiche if desired.