Heat oven to 350°F. Line baking sheets with parchment paper. Set aside.
In a large bowl, whisk together the flour, baking powder, salt, nutmeg and cinnamon.
Melt butter in a small saucepan over medium heat. Cook the butter, stirring constantly, until golden brown, about 8-10 minutes. Immediately remove butter from heat and pour into the bowl of a stand mixer or large bowl. Allow the butter to cool for 10-15 minutes.
Add brown sugar and granulated sugar to brown butter and mix with a stand or handheld electric mixer on medium speed for 2-3 minutes or until fully combined and creamy in texture.
Add egg and 1 teaspoon maple flavoring; beat on low speed until well combined.
Gradually add in flour mixture and mix until a soft dough forms. Stir in walnuts and raisins.
Drop dough by rounded measuring tablespoons onto prepared baking sheets, 1 inch apart. Bake in preheated oven 9 to 11 minutes or until edges are light brown. Allow cookies to cool on cookie sheets for a few minutes before placing them on a cooling rack to cool completely.
Once cookies are cool, prepare the frosting: In medium bowl, combine milk, 2 tablespoons butter, vanilla, 1/2 teaspoon maple flavoring and 1/4 cup powdered sugar.
Beat on low speed with an electric mixer until smooth.
Slowly add in 1 1/2 cups powdered sugar until blended.
Spread frosting onto the top of each cookie using an offset spatula.