In a medium mixing bowl, whisk together the almond flour, arrowroot starch/flour, coconut flour, tapioca starch/flour, baking powder, and salt.
Use a hand held mixer or stand mixer fitted with the whisk attachment - In a small mixing bowl, add the butter and sugar. Beat on medium-high speed to cream the butter and sugar together until they are light and fluffy - 3 to 5 minutes. Note: I use a stand mixer with a 3-quart mixing bowl.
Add the egg and vanilla extract to the butter mix and beat until combined. Switch out the attachment to the regular beater (or paddle if using a stand mixer).
On low speed, spoon in half of the flour mix in increments, and mix until just combined. Add the milk and sour cream and mix to combine. Spoon in the remaining flour mix and mix until just combined.
Spoon half of the coffee cake batter equally into the prepared mini loaf pans. Sprinkle the filling equally over the top batter. Top with the remaining coffee cake batter, then finish by sprinkling equal amounts of the streusel over the top.
Bake the mini coffee cake loaves for 25 to 28 minutes, or until a toothpick inserted in the middle comes out mostly clean. Cool the coffee cakes in the pans set on a wire cooling rack for 10 minutes. Then, using the parchment paper sling, carefully lift the coffee cakes out of the pans to cool on the wire rack completely.