1. Make the crumb topping: In a medium bowl, whisk together flour mix, xanthan gum, and salt. Add sugar and cinnamon; whisk to combine. Add chilled shortening and, using a pastry blender or two knives, blend to form pea-sized crumbs; set aside.

2. Make the cake: Preheat oven to 325 degrees. Coat a 9-by-9-inch square baking pan with shortening and dust with some of the flour mix, tapping out excess.

3. In a large bowl, whisk together flour mix, xanthan gum, baking powder, baking soda, and salt; set aside. In a medium bowl, mix together yogurt and vinegar; set aside. Mix together egg replacer and rice milk; set aside.

4. In the bowl of electric mixer fitted with the paddle attachment, mix together shortening, sugar, egg replacer mixture, and vanilla; beat on medium speed until light and fluffy, about 2 minutes. Add flour mixture in three additions, alternating with the yogurt mixture and beginning and ending with the flour mixture. Beat until just combined, scraping down sides of bowl as necessary.

5. Pour batter into prepared pan, smoothing surface with a spatula. Sprinkle crumb topping over top, using your finger to clump some of the topping together to form larger crumbs. Transfer to oven and bake until the top of cake is golden and springs back when touched lightly, about 45 minutes.

6. Let cool in pan on a rack for about 15 minutes. Invert cake onto a large plate, cover with a cooling rack, and reinvert onto the rack, crumb-side up. Let cool completely. Sift confectioners' sugar over the top and cut into squares.