Heat oven to 350°F.
In a 9x13-inch pan melt the butter in the oven.
Sprinkle the brown sugar evenly over the butter.
Arrange the strawberries and rhubarb on the brown sugar. Press down gently.
Beat the egg whites with the cream of tartar until stiff peaks form.
Beat the egg yolks for two minutes.
Add the remaining ingredients to the egg yolks. Beat for two minutes.
Fold in the egg whites.
Pour the batter over the strawberries and rhubarb.
Bake for 50-60 minutes.
Immediately run knife around sides of pan to loosen cake. Place heatproof serving plate upside down on pan (I used a small baking sheet); turn plate and pan over.
Leave pan over cake 5 minutes so the brown sugar topping can drizzle over cake.
Cool 30 minutes. Serve warm or cool with whipped cream or ice cream.
Store covered in refrigerator.