Preheat the oven to 350 degrees F. Spray a 9-inch round cake pan with nonstick spray.
Heat the butter in a saucepan over medium heat and once melted, whisk in the sugar and the cinnamon until it starts to bubble. Remove it from the heat and pour it in the bottom of the cake pan. Place the blood orange slices over top, covering the bottom of the pan completely.
In a bowl, whisk together the flour, baking powder, salt and cinnamon.
To make the cake batter, place the butter in the bowl of your electric mixer fitted with a paddle attachment and beat on medium speed until creamy. Beat in the sugar and gradually increase the speed to high, beating until fluffy and scraping down the sides if necessary, about 2 to 3 minutes. Beat in each egg until combined. Beat in the vanilla extract and scraped seeds from the vanilla bean.
With the mixer on low speed, add the dry ingredients and stir until the batter just comes together.
Place dollops of the batter all over the oranges and gently spread evenly to cover. Bake the cake for 35 to 40 minutes, or until the top is golden brown and set. Remove from the oven and only let it cool for about 5 minute, just so the fruit settles and stops bubbling. After 5 minutes (no longer!), run a knife along the edges of the cake and place a cake plate over top, then gently flip the cake to remove it from the pan. Here, you can fix any fruit than has slid off or rearrange it if desired. Let the cake cool completely before topping it with the blood orange syrup and serving it with the whipped cream.