Step 1

Arrange a rack in the middle of the oven and heat to 350°F. Line the bottom of a 9-inch springform pan with parchment paper. Coat the paper and sides of the pan with butter; set aside.

Step 2

Place the almond flour, polenta, baking powder, and salt in a medium bowl and whisk to combine; set aside.

Step 3

Place the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment. (Alternatively, use an electric hand mixer and large bowl.) Beat on medium speed until lightened in color, about 3 minutes.

Step 4

With the mixer on medium speed, add 1/3 of the almond flour mixture and beat until incorporated. Beat in 1 egg until incorporated. Continue beating in and alternating the remaining almond flour mixture and eggs in 2 more additions. Stop the mixer and scrape down the sides of the bowl with a rubber spatula.

Step 5

Add the lemon zest and vanilla extract and beat until just combined. Transfer the batter to the pan and spread into an even layer.

Step 6

Bake until the edges of the cake have begun to pull away from the sides of the pan, about 40 minutes. Place the pan on a wire rack and make the syrup.

Step 7

Place the powdered sugar and lemon juice in a small saucepan over low heat and cook, stirring occasionally, until the powdered sugar is completely dissolved and the syrup is warm. Remove from the heat.

Step 8

Using a toothpick, poke holes all over the cake, spacing the holes about 1-inch apart. Slowly drizzle the warm syrup evenly over the cake. Let the cake cool completely, about 1 1/2 hours. Remove the sides of the pan, cut into wedges, and serve with whipped cream if desired.