Grease a 9-inch round cake pan and line the bottom with parchment paper for easy release. Melt 2 tablespoons of butter and spread it evenly in the bottom of the pan.

Sprinkle the brown sugar over the melted butter, then evenly layer the sliced strawberries and rhubarb on top.

In a bowl, whisk together the flour, baking powder, baking soda, and salt.

In a separate large bowl, cream together the softened butter and granulated sugar until light and fluffy.

This takes about 2–3 minutes. Add the egg and vanilla, and mix until fully combined.

Add the dry ingredients in three parts, alternating with the buttermilk. Start and end with the dry mix. Don’t overmix—just stir until it’s all combined.

Gently spread the cake batter over the fruit layer using a spatula. Try not to disturb the fruit too much.

Bake at 350°F (175°C) for 40–45 minutes, or until a toothpick comes out clean from the center. The top will be golden, and you’ll see some bubbling at the edges.

Let the cake cool in the pan for about 10–15 minutes. Run a knife around the edges to loosen it, place a plate or cake stand on top, and flip it carefully.

Peel off the parchment paper and let it cool a bit more before slicing.