Preheat your oven to 180°C (350°F) and grease a 8x8 inch baking tin or line it with parchment paper.
In a large bowl, cream the melted butter and sugar together until light and fluffy.
Add the egg and vanilla extract, and continue to mix until well combined.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the milk, mixing until smooth and well combined.
In a small bowl, mix the melted butter, brown sugar, and cinnamon powder until it forms a paste.
Pour the cake batter into the prepared baking tin and spread it evenly.
Spoon dollops of the cinnamon mixture over the cake batter and swirl it through using a knife to create a marbled effect.
Bake the cake in the preheated oven for 25–30 minutes, or until a skewer inserted into the centre comes out clean.
While the cake is baking, mix the icing sugar, milk, and vanilla extract in a small bowl until smooth.
Once the cake is out of the oven and slightly cooled, drizzle the glaze over the top.
Slice and enjoy your delicious cinnamon roll cake!