Maple Cupcakes With Marshmallow

Ingredients
3 cups all purpose flour
3 teaspoons baking powder
1 teaspoon salt
1/2 cup (1 stick) butter, slightly softened, cut into pads
2 tablespoons vegetable shortening, at room temperature
2 cups pure maple syrup
3 egg yolks
1 large egg
1 1/2 cups whole milk
1 cup walnuts, toasted and coarsely chopped
Preparation method