Preheat oven to 350 degrees F.
Spray a 12 cup muffin tin with non-stick cooking spray, set aside.
Zest two blood oranges. Place zest in a small bowl and set aside.
Juice the zested oranges and set juice aside
Carefully peel and slice remaining two blood oranges into 12 equal slices, each approx 1 cm thick)
In the bowl of a stand mixer, beat butter and sugar until pale and fluffy
Add eggs one at a time, beating for a full minute between each addition
Add orange zest and mix carefully.
The mixture may look a bit curdled, but that's ok
Sift baking powder in with flour, then slowly add to egg and sugar mixture until just combined
Sprinkle 1 teaspoon brown sugar in the bottom of each muffin cavity.
Lay one orange slice on top of the sugar, trimming to fit if necessary
Measure out batter equally among muffin tins. (I used an ice-cream scoop)
Bake for 25-30 minutes
While the cakes are in the oven, combine blood orange juice and 1/2 cup sugar in a small saucepan.
Simmer until the sugar dissolves and the syrup begins to thicken slightly.
When the cakes are done, allow them to sit on the counter for just two to three minutes.
Use an off-set spatula to gently loosen cakes.
Trim any excess cake that rose above the edges of the muffin tin so the cakes are flat.
Take a cooling rack and lay it upside down on the muffin tin.
Flip the muffins over and lift tin to release the cakes.
Cakes should slide out easily.
While cakes are still warm, spoon one tablespoon of syrup over each cake.
Allow the syrup to soak in, then spoon another tablespoon over the top.
Serve cakes warm or at room temperature
Keep any remaining syrup in the fridge and use to sweeten iced-tea or add to ginger ale!