Position a rack in the center of the oven and preheat to 350°F. Line a 9x9-inch pan with parchment, leaving an overhang for easy removal, and lightly grease the parchment.
Make the crumble first: in a small bowl combine 1/2 cup flour, 1/2 cup coconut sugar and cinnamon. Add the chilled cubed coconut oil and use a fork or pastry cutter to work it into coarse crumbs. Refrigerate the crumble while you mix the batter so it firms up.
Whisk the dry cake ingredients in a medium bowl: 1 3/4 cups flour, baking powder, baking soda, salt, pumpkin pie spice and nutmeg until evenly blended.
In a separate large bowl, whisk the eggs until combined. Add pumpkin puree, coconut sugar, Greek yogurt, melted coconut oil and vanilla; whisk until smooth and glossy.
Fold the dry ingredients into the wet mixture with a spatula just until no streaks of flour remain — avoid overmixing. The batter will be thick.
Spoon the batter into the prepared pan and smooth the top. Scatter the chilled crumble evenly over the batter, then press it gently in a few spots so some crumbs sink slightly for a marbled effect.
Bake for 28–32 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs (avoid a completely dry toothpick). If the top browns too quickly, tent with foil for the last 8–10 minutes.
Remove the pan to a wire rack and let the cake cool for about 20 minutes in the pan (this helps it set). Use the parchment overhang to lift the cake out, then cool another 10 minutes before glazing if you prefer a thicker glaze.
Prepare the glaze: whisk powdered sugar with vanilla and 2 teaspoons almond milk; add additional milk, a drop at a time, until it reaches a drizzling consistency. If the cake is still warm, the glaze will sink in slightly and set shiny.
Drizzle the glaze over the cooled (or slightly warm) cake, slice into 12 squares, and serve. Store leftovers tightly covered at room temperature for 2 days or in the fridge up to 5 days.