Preheat oven to 425°F. Spray the doughnut pan with nonstick cooking spray.
In a large mixing bowl, sift together the cake flour, sugar, baking powder and salt. Add the buttermilk, eggs, oil, lemon extract and zest. Beat until just combined. The batter will be thick. Transfer the batter to a piping bag with a 1/2-inch opening. Pipe the batter into the doughnut cavities until approximately 2/3 full.
Bake the doughnuts for 7 to 9 minutes, or until the top of the doughnuts spring back when touched (mine baked in about 7 minutes exactly). Let cool in the pan for 2 minutes. Turn the doughnuts out onto a wire rack to cool completely.
To make the glaze, put the confectioners' sugar in a large bowl. Make a well in the center and add the lemon extract and 2 teaspoons of milk. Stir together with a whisk. Add more milk as needed to form a thick glaze (I used about 6 teaspoons total). The glaze should fall from a spoon in a thick ribbon back into the bowl, leaving a trail that disappears completely by the count of 10.
Pour half of the glaze onto a shallow plate. Use a spoon to swirl in different hues of gel food color (I used rainbow colors: red, orange, yellow, blue, and violet). Dip a doughnut into the glaze and turn it over. Wait a minute for the glaze to spread and settle. If you experience bare spots on the doughnuts, use a spoon to doctor the spot with more glaze. Dip more doughnuts until the color starts to fade. When this happens, pour the remaining glaze onto the plate and swirl in more food color. Dip the remaining doughnuts in the glaze. Let the doughnuts stand until the glaze is set, about 2 hours.
Doughnuts can be stored in an air-tight container overnight, and they will still taste fresh, but the glaze will settle into a wrinkly appearance. If you need to make these ahead of time, I suggest making the cake doughnuts the day before and then glazing them the day you plan to serve them for the nicest presentation.