Preheat oven to 325ºF.

Line an 8-inch square baking pan with foil, allowing the extra foil to hang over the edges of the pan.

Pulse the gingersnaps in a food processor into fine crumbs. Add the brown sugar and pulse again to incorporate.

Melt the butter in the microwave and let it cool for a minute. Then drizzle it into the food processor and pulse about 10 times. The crumbs should be evenly moistened.

Spread the crumb mixture and press evenly in the pan. Bake until golden brown, about 18-20 minutes. Transfer to a wire rack to cool.

While the crust is baking, stir the cream cheese and the lime zest together in a medium bowl.

Add the condensed milk and whisk until all the lumps are gone.

Whisk in the yolk.

Gently whisk in the key lime juice.

Pour the filling into the crust and smooth it out with a silicone spatula. Bake until the filling sets and edges begin to separate from the pan, about 15 to 20 minutes.

Let cool on a wire rack for about an hour. Then cover, and put in the fridge for at least 2 hours or overnight.

To serve, run a knife around the edges and use the foil to lift the whole thing out of the pan. Peel the foil off and cut it into 16 equal pieces. If you like, garnish with powdered sugar.