In a bowl, combine eggs, salt, and vanilla. Mix on medium-high speed for 2 minutes until foamy.

Add sugar and mix on high for 2-3 minutes until the mixture is silky and pale in color.

Add vegetable oil and milk, then mix on low speed just to combine.

In a separate bowl, combine flour and baking powder. Gradually add half of the dry ingredients to the wet ingredients. Mix on low until just combined, using a hand whisk to avoid overmixing. Repeat with the remaining half.

For the filling, combine granulated sugar, brown sugar, and cinnamon powder in a small bowl.

Prepare an 8-inch round cake pan by greasing and lining it with parchment paper. Pour in half of the batter and sprinkle enough cinnamon sugar mixture to cover it. Add the remaining batter on top.

Bake in a preheated oven at 165°C (329°F) for 40 minutes or until the top is golden brown and a toothpick inserted in the center comes out clean.

Allow the cake to cool slightly before removing it from the pan. Brush the top with melted butter and sprinkle the remaining cinnamon sugar mixture over it.

Let the cake cool completely before slicing and serving.