Preheat the oven to 350 °F. Grease a 9 x 13-inch cake pan.
In a large bowl, whisk together flour blend, sugar, baking soda, cocoa powder, salt, cinnamon, nutmeg, and allspice.
In another large bowl, beat together oil, applesauce, raisins and eggs.
Add flour mixture to oil mixture in batches, beating well after each addition. Stir in raisins, and pour into prepared baking pan, spreading out evenly. Bake for 30 to 35 minutes or until a toothpick inserted in the center comes out clean. Cool completely in pan on a wire rack.
Make frosting by beating together butter, vanilla, and milk in a medium bowl until smooth. Add confectioners' sugar, 1 cup at a time, beating well after each addition. Add more sugar, if necessary, to reach a spreadable consistency.
Loosen edges of cake with a sharp knife and cut into 2 lengthwise pieces. Remove 1 piece using 2 metal spatulas, and transfer to a platter. Spread half the frosting over the piece of cake (not on the sides), and place the other half of cake on top. Spread with remaining frosting on top (again not on sides), and decorate by running the tines of a fork lengthwise down the frosting.
Chill for 1 hour before slicing. Keep any leftover cake refrigerated.
*Gluten-Free Flour Blend: Combine 2 cups rice flour, 2/3 cup potato starch, 1/3 cup tapioca flour and 1 teaspoon xanthan gum. Store in container with tight-fitting lid; stir before using.
Makes 3 cups