Step 1

Preheat your oven to 350°F. Line an 8-inch square baking pan with parchment paper and set it aside.

Step 2

The topping must be prepared first, so it can chill.

Step 3

In a medium-size bowl, place all the ingredients in the order listed, mixing to combine after each addition.

Step 4

Cover the bowl that holds the raw topping. Place the bowl in the refrigerator to chill until firm.

Step 5

In a medium-size bowl, place the flour, xanthan gum, cornstarch, baking powder, baking soda, salt, and optional cinnamon, and whisk to combine well. Set the bowl aside.

Step 6

In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, place the butter, sour cream, granulated sugar, milk, and vanilla, and beat well on medium speed until very smooth. Add the eggs, and beat until just combined.

Step 7

Add the dry ingredients to the bowl of wet ingredients and mix by hand until just combined. The batter will be very thick, since our liquid is mainly sour cream.

Step 8

Scrape the batter into the prepared pan and smooth into an even layer with a wet spatula. Press the spatula down in the corners to eliminate any air pockets.

Step 9

Remove the chilled crumble topping from the refrigerator. Uncover the topping, and use the tines of a fork to break up the mixture into irregular pieces.

Step 10

Scatter the crumble pieces evenly on top of the cake batter. Press down gently to help the topping adhere to the batter.

Step 11

Place the cake pan in the center of the preheated oven. Bake for about 40 minutes, until the center does not jiggle in the center when the pan is shaken from side to side gently.

Step 12

A cake tester inserted into the center should come out mostly clean. Do not overbake or the bottom will burn.

Step 13

Remove the pan from the oven and allow the cake to cool to warm room temperature in the pan. Slice into squares and serve.