Grease a 6-quart crockpot with cooking spray and set aside.

Combine milk and lemon juice in a small bowl and let curdle for 510 minutes while preparing the rest of the recipe.

In a medium bowl, combine flours, baking powder, baking soda, salt, and cinnamon.

In another bowl, combine the rest of the wet ingredients, including the milk and lemon juice.

Pour the wet mixture into the dry and stir just to combine.

Do not over-mix.

In a small bowl combine the ingredients for the filling.

Pour half of the batter into the slow cooker, spreading to make sure it reaches all edges.

Sprinkle the filling over the top.

Use the back of a spoon to make sure the entire surface is covered, if necessary.

Pour the rest of the batter over the top, spreading again to make sure it reaches the edges.

Combine the ingredients for the topping in a small bowl.

Use a fork to work in the butter until you have a crumbly mixture.

Sprinkle over the top of the batter.

Turn the crockpot to high and let cook for 1 hour.

After 1 hour, reduce heat to low and cook for another hour.

Check for doneness by inserting a toothpick in the center.

If it comes out clean, turn off the crockpot and crack open the lid to let moisture escape.

Let the cake sit in the crockpot for 1020 minutes to cool before removing.

Remove from crockpot and serve.

Sprinkle with powdered sugar if desired.