Preheat the oven to 350°F (175°C). Line a nine-inch cake pan with parchment paper and lightly grease it. Measure and sift the flour, baking powder, salt, and cinnamon into a large mixing bowl.
In a separate bowl, whisk together the eggs and sugar until the mixture is pale and slightly thickened, about 2-3 minutes. Add the melted butter, vanilla extract, and milk, then stir until well combined.
Gradually add the wet ingredients to the dry ingredients, folding gently with a spatula or mixing on low speed until just combined. The batter should be smooth with no streaks of flour. Do not overmix.
Pour the batter into the prepared cake pan, smoothing the top with a spatula. Lightly tap the pan on the counter to release any air bubbles and ensure even distribution.
Bake in the preheated oven for about 35 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean. The cake should be risen and slightly firm to the touch.
Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack. Let it cool completely before slicing to reveal a moist crumb with a delicate cinnamon swirl visible across the interior.
