Place first four ingredients along with ¼ cup brown sugar, butter and ¼ teaspoon cinnamon in a bowl and mix well with your hands.
Crumble it onto a baking tray.
Bake for 10 to 12 minutes until golden brown. Remove crumble from oven and set aside to cool until ready to use.
Keep oven heated at 350°F.
Next, place eggs and 1¼ cup brown sugar in a stand mixer and mix on high for two minutes.
Then add remaining ¼ teaspoon cinnamon and other remaining ingredients. Mix for another minute.
Line an 8 x 10-inch cake pan with parchment paper and lightly grease.
Pour mix into tray and bake for 30 to 35 minutes. Check with cake tester or toothpick to make sure the cake has set. Allow to completely cool.
Carefully place cake on a cutting board and cut into 12 pieces.
Serve with a couple spoonfuls of crumble and a dollop of whipped cream.
*To make this recipe gluten free, substitute all-purpose flour with ¼ cup Only Oats Gluten-Free Oat Flour.