Preheat your oven to 350°F (175°C) and prepare a 9x9 inch baking pan by buttering it.

Make homemade buttermilk by mixing lemon juice with half and half and letting it sit for 5-10 minutes.

Cream the softened butter and sugar together in a stand mixer until light and fluffy, about 3-5 minutes.

Add the egg and vanilla extract, mixing on low until just combined.

Toss the chopped rhubarb with 1 tablespoon of all-purpose flour to prevent sinking.

Whisk together the remaining flour, baking powder, and salt in another bowl and gradually add to the butter mixture, alternating with the buttermilk.

Gently fold in the flour-coated rhubarb and pour the batter into the prepared baking pan.

Sprinkle sugar on top and bake for 40-45 minutes, checking with a toothpick for doneness.

Allow the cake to cool in the pan for 10-15 minutes, then transfer to a wire rack to cool completely before serving.