Preheat the oven to 350 degrees F. Place the pieces of butter in a 8x8 inch baking pan or 9 inch round pan and melt it in the oven. Sprinkle the brown sugar evenly over the melted butter. Evenly place the strawberries over the brown sugar followed by the rhubarb.
In a small bowl, combine the flour, baking powder, baking soda and salt.
In a large mixing bowl, add the eggs, sugar, oil and vanilla extract and beat to combine. Add the sour cream and beat until combined. Add the milk and flour mixture, alternating between the two and ending with the flour. Beat just until combined.
Pour the batter into the cake pan and bake for 45-55 minutes or until a toothpick inserted into the middle comes out clean. (If the cake is getting to brown on the top before the rest is done, cover it with foil.) Let the cake sit for 5 minutes then run knife around edges and invert the cake onto a plate. (See Note)
Serve with whipped cream or ice cream if desired.
For a prettier appearance, I like to spread on a little more caramel glaze before serving. Here's what you need: 2 tablespoons butter 2 tablespoons brown sugar 2 tablespoons heavy creamPlace the butter, brown sugar and heavy cream in a small saucepan (you can also microwave the ingredients for a minute). Bring to a boil and simmer for 2 minutes. Remove from heat and let it sit for a minute. Pour the glaze evenly over the cake. Let it cool for a couple of minutes before serving.