Preheat the oven to 350 degrees F; position racks in the upper and lower thirds of the oven. Spray three 9-by-2-inch round cake pans with nonstick spray and line the bottom of each with parchment paper. Spray the parchment. Set the pans aside.
In a large bowl, whisk to combine the flour, baking powder and salt; set aside. In a separate bowl or measuring cup, combine the milk, oil, orange juice and zest; set aside.
In a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar and brown sugar together on medium-high speed until light and fluffy, about 8 minutes. Add the eggs, one at a time, beating well after each addition.
Reduce the mixer speed to low and add the dry ingredients in three additions, alternating with the wet ingredients, and beginning and ending with the dry.
Pour one-third of the batter into a medium bowl, add 1 or 2 drops of orange food coloring, and stir to combine. Pour into one of the prepared cake pans.
Pour another third of the batter into the same bowl and add 3 or 4 drops of food coloring. Stir to combine, then pour into the second cake pan.
Add 5 or 6 drops of food coloring to the remaining batter and stir to combine. Pour into the third cake pan.
Bake the cake layers until a toothpick inserted in the center comes out clean, about 25 minutes, rotating the pans halfway through. Cool in the pans on wire racks for 5 minutes. Then invert the cakes, peel off the parchment and let cool completely on wire racks. Trim the tops if domed.
To assemble the cake: Place the darkest cake layer on a cake stand and frost the top completely. Stack the next darkest layer and again frost the top. Stack the final layer and frost the top and sides of the cake. Slice to serve.
Place the chocolate in a large heat-proof bowl set over (not in) a pan of gently simmering water. Heat until the chocolate is melted, then stir until smooth. Remove the bowl from the pan and let the chocolate cool to room temperature.
In a stand mixer with the paddle attachment, beat the butter and confectioners' sugar on medium speed until light and smooth, about 3 minutes. Add the melted chocolate and beat until combined. Add the cream and beat until the frosting is smooth and spreadable.