Happy Pi Day! That’s right; math geeks around the world celebrate the awesome power of that beautifully useful irrational number on the 14th day of the third month by enjoying a slice of delicious pie.

I’ve got gluten-intolerant family visiting this weekend, so I am celebrating Pi Day with a chocolate mousse pie that’s ridiculously simple and efficient. Just make one batter with eggs, chocolate and sugar. Bake half of it into a crust; chill half of it for your filling. The ‘crust’ is moist and almost cake-like, and the mousse is divine. Top it with sweetened whipped cream and it’s almost too good to be true.

This recipe, like many for chocolate mousse, contains raw eggs. I turned to cookbook author David Leibowitz, who gives this advice on his blog:

“Some folks are concerned about raw eggs; if you’re one of them, use pasteurized egg whites, which you should verify are suitable for whipping. Some won’t whip, which I learned the hard way a few years back. When I make anything that uses uncooked eggs, I use the freshest eggs I can get my hands on.”

Chocolate Mousse Pie
Adapted from Maida Heatter’s Book of Great Chocolate Desserts and the Big Girls Small Kitchen blog

Ingredients

• Butter for the pan
• 8 ounces semisweet chocolate (I used Callebaut semisweet chocolate chips)
• 1 tablespoon instant coffee
• 1/4 cup boiling water
• 8 large eggs, separated
• 2/3 cup sugar, divided
• 1 teaspoon vanilla extract
• 1/2 teaspoon salt
• Whipped cream for serving

Preheat the oven to 350 degrees. Butter a 9-inch pie plate well.

Place the chocolate in a microwave-safe bowl. In a separate small bowl, dissolve the coffee in the hot water, then add to the chocolate. Microwave until just smooth in 30-second intervals — it shouldn’t take much more than a minute if you’re using chocolate chips or have cut your chocolate into very small chunks. Set aside to cool. You can also melt the chocolate in the top of a double boiler.

In a medium mixing bowl, use an electric mixer to beat the egg yolks on high speed for 5 minutes, until they’re a light, almost yellow color and have thickened. Stream in half the sugar while beating, then continue to beat on high speed for 5 more minutes. The yolks will be very thick and a little sticky. Add the vanilla and chocolate and beat them in slowly, scraping down the bowl. Set aside and wash and dry the mixer’s beaters well.

Put the egg whites in another mixing bowl and add the salt. With those clean, dry beaters, beat the whites until they hold soft peaks. Add in the remaining 1/3 cup of sugar and beat until the egg whites hold firm peaks.

In two or three small additions, fold about half of the egg whites into the egg yolk-chocolate mixture - no need to be very thorough. Then fold the chocolate into the remaining whites. Fold only until no egg whites show, using big but gentle movements so as not to deflate all those egg whites you just inflated with air.

Handling as little as possible, remove 4 cups of the mousse to a bowl or storage container. Cover and store in the fridge.

Turn the rest of the batter into the pie plate. It may not look like there’s much, but trust me. Gently level it out and place it in the oven to bake. Bake for 25 minutes, then turn off the oven and leave the pie plate in for another 5 minutes. The mousse will rise while it’s baking, and then deflate and settle in the middle, leaving a high rim.

Remove to a rack to cool completely. This will become the crust.

When the baked mousse is completely cool, remove the chilled, unbaked mousse from the fridge and gently mound it in the center of the shell. Spread it to the edges, leaving it slightly higher in the center. Smooth it as gently as you can, handling it as little as possible so you do not lose the air beaten into it.

Refrigerate, covered, at least 2 to 3 hours, or overnight. The tart will set, making it easy to cut.

Serve topped with slightly sweetened whipped cream and shaved chocolate if you desire. Serves 8-12

Jessica Webster leads the Food & Grocery section for AnnArbor.com. Reach her at JessicaWebster@annarbor.com. You also can follow her on Twitter or subscribe to AnnArbor.com's email newsletters.