Preheat oven to 350ºF. Line a 9x13" baking pan with parchment or grease well. In the large bowl of a standing mixer, beat butter and sugar until light and fluffy, about 3 minutes. Beat in egg, then vanilla.

In a separate bowl, whisk together flour, baking soda and salt. With the mixer speed on low, beat in half of flour mixture, followed by yogurt, then remaining flour mixture.

Remove bowl from standing mixer, and fold rhubarb and strawberries into cake batter. Spread into prepared baking pan. In a small bowl, combine remaining 1/4 cup butter with cinnamon and brown sugar. Sprinkle over top of cake. Bake for 50 minutes to 1 hour, until centre is set and springs back when lightly touched. Cool in pan set on top of wire rack. Serve warm or at room temperature.