1. Melt the 3 tablespoons (45g) of butter in a cast iron skillet, or cake pan (see Note). Add the brown sugar and cook while stirring, until the sugar is melted and begins to bubble. Remove from heat and let cool.
2. Once cool, arrange the fruit in a pinwheel design, added berries if desired. Set aside.
3. To make the cake, preheat the oven to 350F. (190C)
4. Beat the 8 tablespoons (115g) of butter and sugar until fluffly. Add the vanilla, then the eggs, one at a time, until smooth.
5. Whisk or sift together the flour, baking powder, and salt.
6. Stir in half of the flour mixture, then the milk, then the remaining dry ingredients. Do not overmix: stir just until the flour is barely incorporated into the batter.
7. Spread the batter over the fruit, then bake for 45 minutes to one hour (depending on the size of the pan, and the thickness of the batter.) The cake is ready when it begins to pull away from the sides of the pan and the center feels just set.
8. Remove from oven, let cool about 20 minutes, then place a cake plate on top, and wearing oven mitts, flip the cake out on to the plate, taking care, as there may be some hot caramel that might escape.