Preheat the oven to 350°F and lightly grease or line an 8x8-inch square baking pan with parchment paper.

In a medium bowl, whisk together the all-purpose flour, almond flour, baking powder, salt, and cinnamon until well combined.

In a separate large bowl, beat the softened butter and sugar substitute with a hand mixer for 2 to 3 minutes until light and slightly fluffy.

Add the eggs to the butter mixture one at a time, beating well after each addition, then mix in the vanilla bean paste and lemon zest.

Add one-third of the dry ingredients to the wet mixture and mix on low just until combined, then add half of the yogurt and mix briefly.

Repeat with another third of the dry ingredients, the remaining yogurt, and then the final third of the dry mixture, mixing only until no dry streaks remain.

Scrape the batter into the prepared pan and smooth the top evenly with an offset spatula or the back of a spoon.

Bake on the center rack for 18 to 20 minutes, until the edges are lightly golden, the center springs back when gently pressed, and a toothpick comes out with a few moist crumbs.

Let the cake cool in the pan on a rack for 10 minutes, then slice into 4 generous squares and serve warm or at room temperature, plain or with a sugar-free fruit compote or a spoonful of Greek yogurt.