Preheat the oven to 350°F (175°C). Grease a cake pan or line it with parchment paper.
In a bowl, mix together the almond flour, erythritol, baking powder, and salt.
In another bowl, beat the eggs, then add melted butter and vanilla extract, mixing until well combined.
Combine the wet and dry ingredients, mixing until fully incorporated.
Fold in the chopped rhubarb gently.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake for 40 minutes or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.