Step 1

In a small saucepan, combine the brown sugar, flour, salt and cinnamon. Whisk until they're well distributed.

Step 2

While whisking, slowly pour in the milk and whisk until the mixture is smooth.

Step 3

Place the saucepan over medium heat and stir continuously (but not too aggressively) for 3-5 minutes.

Step 4

When the mixture thickens and begins to hold streaks, whisk quicker for about another 30-60 seconds. It should be thick like custard.

Step 5

Pour the mixture into a heat safe container, cover it and allow it to cool to room temperature.

Step 6

Combine all the ingredients in a bowl and set aside.

Step 7

Preheat the oven to 350F/177C convection (325F/163C conventional) and line 14 muffin tins with cupcake liners (preferably a light colored pan).*

Step 8

In a medium bowl, or the bowl of a stand mixer, whisk together the cake flour, baking powder, salt and sugar.

Step 9

Add the oil and butter to the dry ingredients and use an electric mixer to beat them together until they're combined.

Step 10

Add the egg and vanilla extract and mix until they're fully combined.

Step 11

Then add the sour cream and milk, mixing just until the batter is smooth.

Step 12

Fill each cupcake liner about 1/3 full of batter (about 30g each), spread 1-2 tsp of cinnamon sugar filling on top and then top with the other half of the batter, filling the muffin tins up to about 3/4 full (another 20g of batter on top).

Step 13

Use a knife to make a gently swirl throughout the cupcakes and bake the cupcakes for about 20 minutes. They should no longer look wet and when you press on the center of the cupcake it should spring back.

Step 14

Remove them from the oven and let them cool in the cupcake pan for 10 minutes, then move them to a cooling rack.

Step 15

Once the custard portion for the buttercream has cooled completely, place the butter into a large bowl or the bowl of a stand mixer.**

Step 16

Use a hand mixer or a stand mixer, with the whisk attachment, to beat the butter at full speed for 5 minutes. Make sure to scrape the edge of the bowl every so often.

Step 17

When the butter is pale and fluffy, add the cooled custard in 3-4 increments, with the mixture running at medium-high speed.

Step 18

Add the vanilla and mix until it's fully combined and smooth.

Step 19

To remove any excess air for smooth piping, you can switch to a paddle attachment and mix on low speed for a few minutes *optional.

Step 20

Use a piping bag fit with a large round piping tip to frost the cupcakes.

Step 21

Use a small sieve to dust with cinnamon.

Step 22

Serve fresh or just slightly chilled.