Preheat your oven to 350 degrees F.

Line an 8 x 8 baking pan with parchment paper.

In a small bowl combine the 1 tbsp cinnamon and 1/3 cup sugar, and set aside.

Use a stand mixer to cream the butter and sugar together until smooth.

Add the eggs and mix well.

Add the vanilla. Mix.

In a separate bowl whisk together the flour, baking powder, baking soda and salt.

In a separate bowl stir together the sour cream and milk.

Add the flour mixture, alternating with the sour cream mixture, to the butter mixture. Combine.

Stir in the chopped strawberries and rhubarb.

Add 1/2 of the batter to the baking pan, smoothing the batter out. Then sprinkle on 1/2 of the cinnamon/sugar mix. 

Spread on the rest of the batter and sprinkle on the remaining the cinnamon/sugar mix.

Use a fork to lightly swirl the cinnamon/sugar mix into the batter (see pictures).

Bake for ~ 50 minutes, or until a toothpick inserted into the centre of the cake comes out clean.

Let the cake cool in the pan for at least 30 minutes before cutting into it.