Caramel and Apples: Butter a 24cm x 8cm round cake tin. Cook the sugar and golden syrup in a large frypan over medium high heat stirring occasionally until the sugar is dissolved and the mixture is amber in colour; about 7 to 10 minutes
Reduce the heat to medium. Carefully add the cream and 4 tablespoons butter then cook while stirring until well combined; about 3 minutes.
Remove the frypan from the heat and let it sit 1 minute. Pour 1 1/2 cups caramel into a glass measuring cup and set aside. Pour the remaining caramel into the prepared cake tin and set aside until set; about 30 minutes.
Cut 3 apples into quarters and cut out the cores. Arrange the pieces curve side down in the tin overlapping them slightly to form a ring about 1cm from the edge. save any bits that don't fit for later.
Halve the remaining whole apples crosswise. Scoop out the seeds with a melon baller or measuring spoon, while leaving the stem on the top half. Dice the bottom and any of the leftover quartered apples into pieces; set aside.
Arrange apple top, stem side down, in the center of the tin. If the quartered apples shift, just push them back into place.
Cake: Preheat oven to 180 degrees C. Sift the flour, baking powder, baking soda, salt, and cinnamon into a medium bowl. In a small bowl, whisk sour cream, orange juice and vanilla.
Using a stand mixer with the paddle attachment, beat the butter and sugar on medium high speed until light and fluffy about 5 minutes.
Crack the eggs into a small bowl. Slip them into the mixer bowl one at a time and beat until the mixture is pale and creamy; about 5 more minutes. Scrape down the bowl and beater with a rubber spatula.
With the mixer on low speed, add half of the sour cream mixture, then half of the flour mixture. Repeat. Turn off mixer and scrape down the bowl with a rubber spatula; finish combining the batter by hand.
Spread the batter over the apples in the tin. Top with the diced apple. Bake until the cake is brown on top and springs back when pressed; about 1 hour . Cool in tin on a rack.
Bring 3cm of water to a simmer in a frypan wide enough to hold the cake tin. Carefully run a sharp knife around the inside of the tin several times, then rest the tin in the water to soften the caramel; about 8 minutes. Remove the tin from the water and dry.
Place a plate on top of the cake then flip over the cake onto the plate. Using pot holders or a towel, wriggle the tin off. Soften the reserved 1 1/2 cups of caramel in the microwave for about 2 minutes. Drizzle the cake with some of the caramel. Slice and serve with the rest.