Preheat the oven to 350 degrees F. Grease one 9-inch round cake pan. I like to also line it with a round of parchment paper for easy removal.
Dice the rhubarb. In a small bowl, toss the rhubarb with the 2 teaspoons of sugar, and set aside.
In a medium bowl, mix together the almond flour, polenta, baking powder, and salt. Set aside.
In a large bowl using a hand beater or a stand mixer, beat the butter and the 2/3 cup sugar until light yellow and fluffy. Add the eggs, one at a time, followed by the coconut milk and the vanilla, beating well after each addition. The mixture may appear slightly curdled; this is normal.
Add the dry ingredients to the wet ingredients (I like to add half at a time), and beat to combine.
Pour the batter into the prepared pan.
Drain the diced rhubarb (the sugar will draw out a good bit of moisture), and sprinkle the rhubarb over the top of the batter.
Bake until a cake tester inserted into the center comes out clean, about 30-35 minutes. The cake should be golden brown and firm to the touch in them center.
Transfer to a rack to cool. Enjoy plain or with whipped cream or a scoop of vanilla ice cream.