Preheat the oven to 350°F (175°C) and set your cake pan aside.
In a saucepan over medium heat, combine 1/2 cup sugar and 3 tablespoons butter. Cook, stirring gently, until the sugar dissolves and the mixture bubbles and turns a deep amber color—this is your caramel. Carefully pour the hot caramel into the bottom of the cake pan, tilting slightly to spread evenly. Sprinkle the blackberries over the caramel in an even layer.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, and salt until well combined.
In another bowl, whisk the sugar, oil, eggs, and vanilla extract until the mixture is smooth and slightly frothy. This helps the batter incorporate air for a light texture.
Pour the wet mixture into the dry ingredients and gently fold them together with a spatula. Add the milk gradually, folding until just combined—be careful not to overmix, as this keeps the cake tender.
Pour the batter over the blackberry layer in the pan, smoothing the top with a spatula for an even surface.
Bake in the preheated oven for 40-45 minutes, or until the top is golden and a toothpick inserted in the center comes out clean. The cake will be bubbling at the edges and fragrant with vanilla and caramel aroma.
Remove the cake from the oven and allow it to cool in the pan for about 15 minutes. Then, carefully invert it onto a serving plate—a quick tap and gentle shaking will release it onto the plate, revealing the caramelized berries on top.
Allow the cake to cool completely or serve slightly warm. The caramel will be sticky and glossy, and the berries will have softened slightly, creating a luscious topping and a tender crumb underneath.
