Preheat your oven to 350°F (175°C). Pour the 1/2 cup of melted butter into a 9-inch round cake pan. Sprinkle the brown sugar evenly over the butter. This will create the caramel base for the fruit.
Spread the cranberries in an even layer over the brown sugar and butter mixture. Sprinkle the orange zest over the cranberries. Gently press down so the fruit is nestled into the sugar.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set this aside. Ensuring your dry ingredients are well-combined prevents any clumps in the final batter.
In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy. This should take about 2-3 minutes with a hand mixer or stand mixer.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract, orange juice, and milk. Don’t worry if the mixture looks slightly curdled at this stage; it will come together once the flour is added.
Gradually add the dry ingredients to the wet mixture. Mix on low speed just until combined. Be careful not to overmix, as this can make the cake tough. Pour the batter over the cranberries in the pan and smooth the top with a spatula.
Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for exactly 10 minutes. This timing is crucial for the “upside down” part to work.
Run a knife around the edge of the pan to loosen the cake. Place a serving plate upside down over the pan. Using oven mitts, quickly and carefully flip both the pan and the plate. Let it sit for a minute so the caramel drips down, then gently lift the pan.