# Happy Tau Day!

Today, 6-28, is Tau Day. Who knew? Tau is like pi: Pi (π) is the ratio of a circle’s circumference to its diameter; tau (τ) is the ratio of a circle’s circumference to its radius. The radius is half the diameter, and τ has only half as many legs as π. Pi, briefly, is 3.14; Pi(e) Day is celebrated on March 14 with pie. Tau, briefly, is 6.28, so I thought I’d celebrate with a τarτ.

Coincidentally, yesterday in the Serious Eats newsletter there was a slideshow about spices to use with fruit. Apparently, aji panca is just the thing to perk up blueberries. Aji panca is a chili pepper, and I find chili peppers painful to eat. However, you can temper the heat with dairy, so my Tau Day tart is blueberry custard tart with aji panca.

Aji panca, if you can find it, is available as fruit, paste, and powder. I got aji panca powder at Christina’s in Inman Square. They had a good-sized jar for \$3.27. You can find aji panca in stores that carry Peruvian groceries, and Amazon has it in all three forms.

A jar of aji panca powder from Christina's

Blueberry Custard Tart with Aji Panca
Pâte Sucrée Crust
7 oz all-purpose flour (about 1½ cups by the spoon-and-sweep method)
3 Tablespoons granulated sugar
1/8 teaspoon baking powder
1 oz (2 Tablespoons) cold vegetable shortening
2½ oz (4 Tablespoons) cold butter cut into chunks
1 egg
1 teaspoon vanilla
1 teaspoon cold water

In a food processor fitted with the metal blade, pulse the flour, sugar, and baking powder to combine. Add the shortening and butter and pulse until the mixture resembles coarse cornmeal. Add the egg, vanilla, and water, and pulse until the dough comes together into a ball.

Roll out the dough and fit it into a 10-inch tart pan. (You could also use a 9-inch pie plate.) Put the tart shell in the freezer (or refrigerator) until you’re ready to fill it.

Preheat the oven to 375℉.

Blueberry Custard Filling
⅔ cup sugar
½ teaspoons aji panca powder
2 eggs
8 oz (1 cup) whole milk
1 pint (maybe a little more) fresh blueberries (1½ to 2 pints if you’re using a 9-inch pie plate)

Whisk together the sugar and aji panca powder.

Sugar and aji panca powder (not whisked together)

Whisk in the eggs.

Eggs whisked in

Whisk in the milk.

Milk whisked in

Distribute the berries over the tart shell.

Blueberries distributed in the tart shell

That 1 pint didn’t quite cover the shell, so I added more from a second pint. (You don’t need to cover the shell, though.) Pour the custard over the berries.

Bake at 375℉ for 15 minutes. Turn the temperature down to 325℉ and continue baking until the custard is set but still jiggles in the center, about another 45 minutes.

Cool the tart on a rack.

Tart cooling on a rack

Remove the tart from the pan. Serve and enjoy.

A slice of tart

I found the aji panca gave the tart a little nip, the way cinnamon does when you use the right amount, without tasting like hot pepper. Possibly it could take 1 teaspoon because the milk takes the edge off the pepper. It would be interesting to see how it works (instead of cinnamon) in a regular blueberry pie.