1. Preheat the oven to 350 F and line an 8″x8″ standard baking pan with parchment paper, allowing the ends of the paper to overhang on the sides. Whisk together the melted butter with the brown sugar, and then pour this mixture all over the bottom of the prepared pan.
2. Place the peeled orange slices, side by side, into the bottom of the pan, on top of the glaze. Fill in all of the gaps with small pieces of orange (see blog post above for photos and directions).
3. In a large mixing bowl, whisk together the flour, baking powder, baking soda and salt. Set aside. In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld electric mixer, beat the softened butter with both sugars, on medium-high, until light and fluffy, about 1 minute. Add the egg and vanilla and mix again. Add half of the dry ingredients, and mix together on low, slowly, for about 30 seconds. Add the milk, and the remaining dry ingredients, and mix again on low for 15-20 seconds. Add the orange juice, and mix again, on low until all ingredients are incorporated. Pour the cake batter on top of the pan with the prepared oranges, spreading it with a spatula to make it even. Place the pan in the preheated oven, and bake for 40-45 minutes. Check the cake after 40 minutes by inserting a toothpick in the center; if the toothpick comes out clean, the cake is done. If there is batter on the stick, bake for another 5 minutes. Remove the cake, and allow it to cool for 10-15 minutes before inverting the cake on to a cake stand or plate.***
4. Allow the cake to cool completely, before slicing and serving. Enjoy!

The cake can be stored in an airtight container at room temperature, for 2-3 days, or 4-5 days in the fridge. I would not recommend freezing the prepared cake.