Heat the oven to 325°F with a rack in the middle position. Butter the bottom and sides of an 8-inch-round cake pan. Line the bottom with kitchen parchment, then butter the parchment. In a medium bowl, whisk together the almond flour, all-purpose flour and baking powder.

In the bowl of a stand mixer with a paddle attachment, mix the sugar, both zests and the salt on low until the sugar appears moistened and clumps, about 1 minute.

Add the butter and mix on medium-low until the mixture is cohesive. Increase the mixer to medium-high and beat until pale and fluffy, about 3 minutes.

Reduce the mixer to low and add the eggs, one at a time, scraping down the bowl after each addition.

Add the dry ingredients and mix on low just until combined, 10 to 15 seconds. Use a silicone spatula to fold the batter until no streaks of flour remain. (The batter will be very thick.)

Scrape the batter into the prepared pan. Spread into an even layer, then sprinkle the almonds on top.

Bake until the cake is golden brown and the center feels firm when lightly pressed, about 55 minutes, rotating the pan halfway through.

Meanwhile, make the syrup. In a small saucepan over medium, combine all ingredients. Bring to a simmer, stirring until the sugar dissolves. Remove from heat and let the syrup steep until needed.

When the cake is done, return the syrup to a simmer over medium heat. Use a toothpick or skewer to poke holes all over the cake's surface. Brush all of the hot syrup evenly onto the hot cake. Cool the cake in the pan until barely warm to the touch, about 30 minutes.

Lightly butter a sheet of kitchen parchment, then place it on the cake buttered side down. Invert a large plate on top of the parchment, then invert the plate and cake pan together. Lift off the pan and remove the parchment round. Re-invert the cake onto a serving platter and let cool completely.