Preheat your oven to 170°C and line your cake pans with parchment paper.
In a large bowl, whisk together the flour, sugar, baking powder, and salt until evenly mixed.
Add the egg, milk, and vanilla extract and whisk until a thick batter forms. A few small lumps are perfectly fine at this stage.
Pour in the melted butter and whisk again until the batter becomes smooth and fully combined.
Make the brown sugar and cinnamon butter: Mix softened butter, brown sugar, cinnamon powder, and sea salt until soft and spreadable.
Transfer the cake batter into the prepared pans, then spoon or pipe small amounts of the cinnamon butter over the batter, and gently swirl it using a knife or spatula. Save a little filling for assembling the cake later.
Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool completely before slicing and layering.