The picture shows it with frosting on the outside too, but I've never bothered with that. If you can make it look that good though, go for it! Below is how Clark does it. :-)
Beat together
3 eggs
1 cup sugar
2/3 cup pumpkin
1 Tbsp lemon juice
Sift together
3/4 cup flour
1 Tbsp baking powder
2 tsp cinnamon
1 tsp ginger
1/2 tsp nutmeg
1/2 tsp salt
Add dry ingredients to pumpkin mixture and blend well. Pour into a well greased and floured 10x15 cookie sheet. Sprinkle 1/2 cup chopped nuts on the top, if desired. Bake at 375 degrees for 15 minutes, or until the sides of the cake pull away from the edges of the pan.
While cake is baking prepare a towel or baking cloth (high quality paper towels can be used) by sprinkling with powdered sugar. When cake is removed from the oven, immediately
invert pan so cake falls onto powdered cloth. Roll cake up from the short end with towel while it is still hot. Place in refrigerator for 20 minutes or let cool to room temp outside of fridge for about 2 hours.
While cake is cooling, prepare filling
1 cup powdered sugar
1/4 cup butter (room temp)
8 oz cream cheese (room temp)
1 tsp vanilla
Blend all ingredients until smooth.
When cake is cool, unroll cake and spread filling in an even layer over the whole cake. Roll cake back up (without towel) and refrigerate until firm. Slice and serve. Cake may be frozen and keep for several weeks.