In a saucepan over medium heat melt the butter. Continue to heat, occasionally swirling the pan, until the butter becomes fragrant & browned. Keep a close eye as the butter can go from brown butter to burnt butter very quickly. Let cool slightly.
In a small mixing bowl whisk together the flour, baking powder, salt, sugar, lemon zest & cinnamon until combined. Stir in the browned butter & mix until a crumbly mixture is formed. Set aside until ready to assemble.
In a medium mixing bowl stir together rhubarb, strawberries, lemon juice, vanilla, sugar & cornstarch. Let stand for 15 minutes.
Transfer fruit to an 8 by 8-inch baking dish. Sprinkle with the crumble topping.
Bake at 375(f) degrees until bubbly & golden, about 45 minutes. Serve warm or at room temperature with ice cream.
