Preheat the oven to 180C/350F/Gas 4. Line a 12-cup muffin tin with muffin cases, and set aside.
Whisk together the flour, baking powder, baking soda, and salt. Set aside.
Whisk together the eggs. Add the sugar, and whisk until combined. Add the vanilla extract, butter, and yogurt, and whisk until combined. Fold in the dry ingredients, then fold in the popcorn.
Divide the batter evenly between the muffin cases, filling only 2/3 full. Place a few extra popcorn pieces on top, if desired.
Bake for 15 - 20 minutes, or until a toothpick inserted into the centre comes out clean. Allow to cool for 5 minutes in the pan, before transferring them to a wire rack to cool completely before frosting.
For the Frosting
Using a handheld or stand mixer fitted with the paddle attachment, beat the butter on high until pale and creamy.
Add half of the icing sugar and, starting on low speed and working up to high, beat until smooth and combined. Repeat with the remaining icing sugar.
Add the milk, and beat on high until light and fluffy. Add more milk if too thick, or more icing sugar if too thin.
On low speed, mix in the crushed popcorn until evenly incorporated.
Pipe or spread the frosting onto the cooled cupcakes. Top each cupcake with toffee popcorn, and enjoy!
Notes
*Make your own cake flour! Measure out 1 and 1/2 cups (190g) of plain/all-purpose flour, remove 3 tablespoons, and replace with 3 tablespoons of cornflour/cornstarch. Sift well.Leftover cupcakes can be stored, in an airtight container, in the fridge for up to 3 days.