Spiced apple walnut cupcakes with brown sugar buttercream

These spiced apple walnut cupcakes, with brown sugar butter cream frosting, and dulce de leche filling, hit the right spot during the holidays.
It was my first time making butter cream, and despite my apprehensions, it worked. I'm not a big fan of butter cream but this brown sugar version is a nice alternative, it masks the strong taste of butter and is easy to work with.

As the pastry chef who once taught me put it, "butter cream is virtually indestructible." The trick is to know when the egg white sand sugar aren't tripling in size and to start over before adding the more-expensive ingredient - the butter.
As for the decorations, I used red sprinkles, dragees, and couverture chocolate, piped into heart shapes.

Cupcake ingredients (adapted from recipe provided at Bonnie Gordon's School of Confectionary Arts):

1 cup all-purpose flour
3/4 tsp baking powder
1/2 tsp cinnamon
1/4 tsp baking soda
1/4 tsp ground ginger
1/4 tsp allspice
1/4 tsp nutmeg
1/4 tsp salt
3/4 cup sugar
1/4 cup butter
1 egg
1/2 tsp vanilla
1/2 cup unsweetened applesauce
2 tbsp plain yogurt
1/4 cup finely chopped walnuts
1/2 of a Gala apple peeled and diced

Directions:
Sift all dry ingredients into small bowl, whisk together and set aside.

Using paddle attachment in bowl of electric mixer, beat together sugar and butter until well combined. Add egg and beat well, add vanilla.

In small bowl, mix together applesauce and yogurt.

Alternately beat flour mixture and yogurt mixture into butter mixture, making three additions of flour and two of yogurt mixture, making sure to scrape down the sides of the bowl after each addition.

Beat batter until smooth. Fold chopped walnut and apple by hand.

Spoon batter into prepared muffin tins and bake at 300-325 for approximately 20 minutes.

Butter cream frosting ingredients:
200 g brown sugar
5 egg whites
2 cups unsalted soft butter
pinch of salt
2 tsp vanilla

Directions:
Warm egg whites, sugar and salt together, whisking over pot of simmering water. Once sugar has dissolved, whip in electric mixer until cool. Meringue should triple in volume.

Add soft butter in chunks, with continual mixing. When butter is fully incorporated and curdled look changes to smooth satin look the butter cream is ready to use.