Preheat the oven to 350F Lightly butter a 9×9 square baking pan.

Cream butter and sugar in a stand mixer, or with electric beaters, until fluffy and pale yellow. Add the egg and vanilla, scraping down the bowl as necessary.

Pour in buttermilk and mix to incorporate

Take 1 Tablespoon of flour and toss the sliced Rhubarb in it.

Combine remaining flour, bakind powder and salt

Add half the dry ingredients and mix gently. Add remaining dry ingredients and mix until just incorporated, dont overmix.

Fold coated rhubarb into the batter. It will be thick.

Spread batter into prepared pan and sprinkle with sanding sugar.

Bake for 40-45 minutes until a toothpick comes out clean. Remove from oven, cool for 5 minutes before inverting onto a rack to fully cool.

You can slice this cake when it is still warm.