Prepare the Pan: Preheat the oven to 350°F (175°C). Grease a 9-inch round cake pan and line the bottom with parchment paper.
Mix the Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
Beat the Eggs and Sugar: In a large mixing bowl, beat the eggs and granulated sugar until pale, thick, and fluffy, about 3–5 minutes.
Add Vanilla and Butter: Add the vanilla extract and melted butter. Mix gently until combined.
Combine the Batter: Gradually add the dry ingredients, alternating with the milk. Gently fold until the batter is smooth and just combined. Avoid overmixing.
Bake: Pour the batter into the prepared cake pan and smooth the top. Bake for 30–35 minutes, or until golden and a toothpick inserted into the center comes out clean.
Cool the Cake: Allow the cake to cool in the pan for about 10 minutes. Transfer it to a wire rack and let it cool slightly before adding the caramel.
Make the Caramel: Add the brown sugar, butter, and heavy cream to a small saucepan over medium-low heat. Stir continuously until the sugar dissolves and the mixture becomes smooth and glossy.
Add Vanilla: Remove the caramel from the heat and stir in the vanilla extract and salt, if using. Allow it to cool for a few minutes so it thickens slightly.
Top the Cake: Place the sponge cake on a serving plate. Pour the warm caramel over the top and gently spread it toward the edges, allowing some caramel to drip down the sides.
Slice and Serve: Let the caramel settle for several minutes before slicing. Serve the cake slightly warm or at room temperature.