Cream butter and sugar until light and fluffy.
Mix in eggs and vanilla.
In a separate bowl, mix flour, salt and baking powder together.
Slowly add flour mixture to butter mixture.
Mix in milk. Set aside.
In a 10 inch cake pan, melt 2 tablespoons of butter. Swirl butter around so it covers all surfaces.
Sprinkle brown sugar over melted butter.
Layer strawberries on top of brown sugar.
Sprinkle rhubarb over strawberries.
Carefully spread cake batter over rhubarb. The cake batter is fairly thick.
Bake at 350 degrees for 60 minutes until cake is done. Test with a toothpick inserted in the center. If if comes out clean, it is done.
Rest on a cooling rack for 10 minutes.
While still warm, run a knife around outside of cake. Place a large plate over the top of the cake. Carefully, but quickly flip cake pan and plate over. Once the cake falls onto plate, remove cake pan. Continue cooling on plate.