Bring the refrigerated ingredients out to lose their chill, then arrange the cake and crumble ingredients where you can reach them. The butter should feel soft enough to cream without looking melted.

Heat the oven to 350°F, grease a 9x9 square dish or a 9x11 casserole dish, and line it with parchment that extends above the sides if you want a lifting sling. The parchment should lie flat against the greased pan so the batter can spread evenly.

Cream the softened butter with the cake portion of monk fruit sweetener, then beat in the eggs one at a time before mixing in the sour cream. The mixture should look evenly combined, even if it loses some of its initial fluffiness.

Add the cake portion of almond flour, baking powder, baking soda, xanthan gum, and nutmeg; mix just until no dry patches remain. The finished batter should be thick and spreadable.

Mix the remaining almond flour and sweetener with the cinnamon, salt, and optional pecans in a separate bowl, then stir in the melted butter. The streusel should form coarse, loose clusters rather than a smooth paste.

Spread half the batter in the prepared pan, scatter half the streusel over it, then gently spread on the remaining batter and finish with the remaining streusel. The center crumbs should stay in a distinct layer, and the top crumbs should sit loosely on the batter.

Bake for 35 to 40 minutes, checking that the crumbs are lightly browned and a toothpick inserted into the cake comes out clean. The edges should look golden, with no wet batter clinging to the toothpick.

Let the cake cool in the pan before lifting it by the parchment, if used, and cutting it into slices. The crumb should feel settled enough to cut without breaking apart.