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Here's a recipe specifically for high altitudes
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Pineapple Upside Down Cake is a challenge. When I lived at 4700' I had the same problem for a while. Here are a couple of thoughts. First, PAUSD cake is traditionally made with what is technically a quick cake or bread method, meaning that it contains oil rather than butter. I used to use a recipe from one of The Silver Palate cookbooks, but even when I adjusted for altitude, I was unhappy with the results. Once I switched to a butter cake recipe, I finally began to produce PAUSDs I was willing to let someone outside the family see. Try this, from The Gourmet Cookbook:
Using the paddle attachment, beat together until pale and fluffy (as in almost white):
3 ounces of room temperature butter
3/4 cup granulated sugar
1 teaspoon vanilla extract
Add one at a time, allowing each to be fully incorporated before adding the next:
2 large eggs
The butter and sugar set up the emulsion which takes place when you add the eggs. The fats in the yolks help emulsify the water in the whites.
Sift together onto a sheet of parchment:
1.5 cups cake flour (Softasilk brand is commonly available boxed in the baking aisle of grocery stores)
2 teaspoons baking powder
1/2 teaspoon salt
Measure 4 ounces of whole milk
Tip the sheet of parchment, allowing about 1/3 of the flour mixture to flow into your mixer bowl. Add 2 ounces of the milk. Mix on low speed just until the flour has been hydrated.
Again tip the parchment sheet, allowing half of the remaining flour mixture to flow into the bowl. Add the remaining milk. Mix on low speed just until the flour has been hydrated.
Finally, add the last of the flower mixture to the bowl. Mix on low speed for one minute beyond when you can see that the flour has been hydrated.
NOW, let's talk about adjusting for altitude. I wrote the High Altitude Baking section for the Bakers Dozen Cookbook, a professional bakers' group I belong to. Essentially, you need to increase the protein in your cake, in the form of flour and egg, in order to better support it in its risen state, and also decrease the baking powder because there is quite literally less atmospheric pressure on the surface of you cake at altitudes above 2000 feet. I took the liberty of adjusting the egg for you; the original recipe calls for 2 egg yolks. If you use 2 whole eggs instead, you've taken care of adding more protein there because the abundance of protein in eggs is contained in the white. Flour: for your elevation, increase it by 5.5%, or 1.58 cups. Be sensible and round up to 1/75 cups. Decrease your baking powder by 36% - to 1.28 teaspoons, which rounds nicely to 1.25 teaspoons.
Last, increase your baking temperature by 25 degrees, which helps insure that the protein molecules surrounding tiny pockets of carbon dioxide produced by the baking powder remain encapsulated and in a beautifully risen state.
Prepare your topping as you would from your recipe. Pour your batter in on top of it. Bake your cake. Set a timer for 30 minutes and give it a test by bouncing your finger on top of the very center. If it feels liquidy, let the cake bake for another 5 to 10 minutes, then check it again. If you can't resist sticking a toothpick into the center just to be sure, by all means go right ahead.
When your cake is done, remove it from the oven and allow it to cool on a rack until you can comfortably handle the cake pan with bare hands. Run a narrow spatula or paring knife all around the edge, set a cake cardboard or serving plate on the top, and turn it all upside down. Use a rubber spatula to scrape out any topping that lingers inside the pan.
Be proud of what you have done!