Combine marshmallows and water in bowl. Microwave, stirring every 30 seconds, 1 1/2 minutes or until melted and smooth. Add 4 cups powdered sugar and 1/4 cup butter; stir, using buttered hands, until smooth.
Remove 1/3 fondant; knead in green food color until color is evenly distributed. Knead orange food color into remaining fondant. Divide orange fondant in half. Wrap each orange fondant in plastic food wrap; refrigerate until ready to roll out.
Create 2 sets of carrot leaf stems with green fondant by pushing through garlic press onto cutting board or waxed paper. Set aside.
Heat oven to 350ºF. Grease and flour 9x5-inch loaf pan; set aside.
Combine flour, sugar, brown sugar, salt, cinnamon, baking powder, baking soda, allspice and nutmeg in bowl. Add carrots; toss to coat. Set aside.
Combine 1/2 cup butter, sour cream, eggs and 1 teaspoon vanilla in bowl; beat until well mixed. Add butter mixture to flour mixture; stir until well mixed. Stir in pecans. Pour into prepared pan. Bake 50-55 minutes or until toothpick inserted in center comes out clean. Cool in pan 10 minutes. Remove to cooling rack; cool completely.
Combine all frosting ingredients in bowl. Beat until well mixed and fluffy.
Place toothpick in center along edge of 1 short end of cake. Cut cake from each opposite corner, diagonally, to toothpick. Place larger triangle onto platter. Place 2 remaining triangles, cut-sides together to form second carrot. Place onto platter next to other cake. Frost cakes; refrigerate until firm.
Roll out half of orange fondant on board dusted with powdered sugar to 3/16-inch thickness creating trapezoid-shape measuring 11 inches long, 9 inches across bottom and 5 inches across top. Cover 1 cake triangle with orange fondant, overlapping slightly. Trim along bottom of cake with pizza cutter or sharp knife to remove excess. Repeat with remaining orange fondant and cake.
Position carrot leaf stems in center of wide end of each carrot, pressing in slightly. Use back of butter knife to create ridges in carrot on top.