Purée strawberries and sugar in food processor, strain though fine mesh strainer and set aside.
Butter a 9-inch square cake pan. Melt 6 Tbsp butter in medium saucepan over medium heat. Add brown sugar, salt and vanilla extract and cook, whisking until mixture comes together, about 3 to 4 minutes. Pour mixture into cake pan, distributing evenly. Arrange rhubarb slices in a decorative pattern over mixture. Spoon 4 Tbsp strawberry purée evenly over rhubarb, reserving remainder of purée for later garnish. Set cake pan aside.
Preheat oven to 350° F.
In a mixing bowl, sift together flour, baking powder, baking soda and salt and set aside.
In separate bowl, mix together sour cream and milk until well combined and set aside.
In separate bowl, with an electric mixer beat together butter, sugar and orange zest until light and fluffy. Add eggs one at a time, beating well after each addition (and scraping sides of bowl with rubber spatula if necessary). Add vanilla extract and mix well.
Reduce speed to low and add dry ingredients and milk mixture, alternately in three or four batches, beginning and ending with dry ingredients, until batter is just smooth. Gently pour batter into cake pan and spread evenly atop fruit, being careful not to disturb fruit. Bake until top is golden and wooden pick inserted into cake center comes out clean, about 55 to 65 minutes.
Cool cake on rack for 5 minutes (no longer), invert onto serving plate and reposition fruit as necessary.
Serve warm or at room temperature with fresh strawberries, reserved strawberry sauce and either whipped cream or vanilla ice cream.Makes 4 Source: