Preheat oven to 350°F (175°C) and line an 8-inch square baking pan with parchment paper.
Whisk whole wheat pastry flour, poppy seeds, baking powder, baking soda, and salt in a primary mixing bowl.
In a secondary bowl, emulsify Greek yogurt, maple syrup, eggs, coconut oil, lemon zest, lemon juice, and vanilla extract.
Integrate the liquid mixture into the dry ingredients using a spatula until no dry pockets remain, avoiding over-agitation of gluten.
Combine almond flour, oats, 2 tablespoons maple syrup, and 1 tablespoon coconut oil in a small vessel to create the crumb texture.
Transfer cake batter to the prepared pan and level the surface.
Distribute the crumb topping evenly across the batter surface.
Bake for 35 minutes or until an internal temperature of approximately 200°F (93°C) is reached or a tester comes out clean.
Cool completely on a wire rack before portioning into 12 squares.