Strawberry Rhubarb Upside-Down Cake

Preheat oven to 350°F. Grease the inside of an 8 or 9-inch round cake pan with nonstick baking spray and line with a parchment paper round (optional). melted butter, making sure to butter all sides.
Add melted butter and sprinkle the bottom of pan with brown sugar and spread strawberries and rhubarb on top. You can arrange them into a fun pattern, or simply scatter them into a single layer. (Exact amount of fruit needed may depend on your pattern and size of cake pan, but just make sure the entire bottom of the pan is covered.)
In bowl of stand mixer fitted with paddle attachment, or in bowl with hand-held mixer, beat butter and granulated sugar until light and creamy. Add egg and vanilla and mix until incorporated.
In separate bowl, combine flour, baking powder, baking soda and salt. Add to butter mixture in two additions, alternating with buttermilk and mixing just until combined, scraping down bowl as necessary.
Spread batter on top of fruit and bake until cake is lightly browned on top and the center springs back when gently pressed, about 25-35 minutes (time will vary depending on if you use an 8 or 9-inch pan).
Let cool 5-10 minutes, then run knife around edges and invert onto plate while the cake is still hot. Cool completely, then serve slices with whipped cream or vanilla ice cream.