Preheat oven to 350°. Line an 8" round cake pan with a parchment round, then spray the sides with nonstick spray; set aside.
If you haven't already, make the pistachio flour. To do so, place whole pistachios into a small blender or food processor and blend until they're a flour-like consistency (you may need to stop and scrape down the sides 1-2 times).
In a medium bowl, stir the all purpose flour, pistachio flour, sugar, baking powder, baking soda, and salt together.
In a large bowl, whisk the butter, egg, milk, applesauce, vanilla, and almond extract together. Then, pour the dry ingredients into the wet and stir to combine, just until the flour disappears.
Pour the cake batter into the prepared cake pan, then bake the cake for 35-40 minutes or until a toothpick comes out clean. Leave the cake in the pan for 10 minutes, then invert it into a wire rack. Wait for the cake to cool completely before icing it.
For the icing, whisk the mascarpone, butter and vanilla together with an electric mixer, then add the powdered sugar and continue whisking until the icing is smooth.
To assemble: Place a dollop of icing on top of the cake, then use an offset knife or the back of a spoon to smooth it out. Repeat this process until all of the icing is gone. Add whole or chopped pistachios on top. Slice and enjoy!