Preheat the oven to 350F/177C convection (325F/163C conventional) and line 14 muffin tins with cupcake liners (preferably a light colored pan).*
In a medium bowl, or the bowl of a stand mixer, whisk together the cake flour, baking powder, salt and sugar.
1 1/2 cup (172g) cake flour, 1/2 Tbsp baking powder, 1/4 tsp fine sea salt, 1 cup (200g) granulated sugar
Add the oil and butter to the dry ingredients and use an electric mixer to beat them together until they're combined.
1/4 cup (60mL) canola oil, 1/4 cup (56g) unsalted butter
Add the egg and vanilla extract and mix until they're fully combined.
1 large egg, 2 tsp vanilla bean paste
Then add the sour cream and milk, mixing just until the batter is smooth.
1/2 cup (113g) sour cream, 1/3 cup (78mL) whole milk
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).
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.
Remove them from the oven and let them cool in the cupcake pan for 10 minutes, then move them to a cooling rack.