Preheat oven to 375°F (190°C). Lightly grease a doughnut pan with cooking spray.

In a medium bowl, whisk together the flour, cinnamon, nutmeg, baking powder, and baking soda. Set aside.

In a large bowl, cream together the softened butter, brown sugar, and salt until well combined.

Add the egg, vanilla extract, and plain yogurt. Beat until smooth.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.

Fold in the crushed pineapple and shredded carrots with a spatula.

Spoon the batter into a piping bag or a resealable plastic bag with the corner snipped off. Pipe the batter into the prepared doughnut pan, filling each cavity about halfway.

Bake for 10 minutes.

Let the doughnuts cool in the pan for 10 minutes before inverting them onto a wire rack to cool completely.

While the doughnuts are cooling, prepare the frosting. In a bowl, cream together the softened butter and cream cheese until smooth and creamy.

Add the reserved pineapple juice and powdered sugar. Beat until well combined and smooth.

To toast the walnuts, add them to a dry skillet over medium heat. Cook, stirring frequently, for 3-5 minutes until fragrant. Let cool.

Once the doughnuts are completely cooled, frost them with the pineapple cream cheese frosting and sprinkle with toasted walnuts.