Pre-heat the oven to 170 degrees celsius and line a cupcake tray with cupcake liners.
In a large bowl, add room-temperature butter and caster sugar. Whip on high speed with an electric mixer for 3–5 minutes until pale and fluffy. (Make sure the butter isn't too cold, or it won't cream properly.)
Add room-temperature eggs and vanilla extract. Beat for another 2–3 minutes until smooth and fully combined. (No streaks of egg should remain.)
Add in the flour, baking powder, cornflour (or extra flour if substituting), and salt. Gently fold the dry ingredients into the batter with a spatula first, then mix briefly on low speed. (Avoid overmixing to keep the cupcakes tender.)
Divide the batter into cupcake liners, filling each about halfway.
Pipe a swirl of the cinnamon filling into the center of each cupcake, pushing the piping tip slightly into the batter so the filling stays inside while baking.
Bake for 20–25 minutes and let the cupcakes cool completely before frosting.