Preheat your oven: Begin by preheating your oven to 350°F (175°C). Grease your cake pan with butter or cooking spray, or line it with parchment paper.

Mix dry ingredients: In a medium bowl, whisk together the flour, baking powder, , salt, and cinnamon (if using). Set this aside for later.

Cream the butter and sugar: In a large bowl, use a hand mixer or stand mixer to cream together the softened butter and granulated sugar until the mixture is light and fluffy. This should take about 3-4 minutes.

Add the egg and vanilla: Beat in the egg and vanilla extract until well combined.

Incorporate the sour cream: Add the sour cream to the wet mixture and stir until smooth.

Combine with dry ingredients: Gradually add the dry ingredients to the wet mixture, mixing until just combined. Be careful not to overmix.

Fold in the rhubarb: Gently fold the chopped rhubarb into the batter, ensuring it’s evenly distributed. The rhubarb will add moisture and a lovely tartness to the cake.

Bake: Pour the batter into the prepared pan and spread it evenly. Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean. Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.

Melt the butter: While the cake is cooling, prepare the butter sauce. In a small saucepan, melt the butter over medium heat.

Add cream and sugar: Once the butter is melted, stir in the heavy cream and brown sugar. Bring the mixture to a simmer and cook for 3-4 minutes, stirring constantly, until the sauce thickens slightly.

Finish the sauce: Remove the saucepan from the heat and stir in the vanilla extract and a pinch of salt. The butter sauce should be smooth, rich, and creamy.