Preheat oven to 350°F. Line an 8x8-inch square baking pan with parchment paper or greased foil. Set aside.
Mix together your ground flaxseed and water, and set aside for at least 5 minutes to gel to create your flax egg. Set aside for later
Add the 3 ½ tbsp oat flour, 3 ½ tbsp melted plant-based butter, ¼ cup cane sugar, pecans, and ¼ tsp cinnamon into a bowl. Using a fork, stir and fold until well incorporated. This is the streusel topping that will be added to the coffee cake later. Set aside.
Next, add your melted butter and sugar to a mixing bowl and cream together. This will take about 2 to 3 minutes. Then add the pumpkin, maple syrup, flax egg, and vanilla into a mixing bowl. Whisk until well incorporated.
Add the oat flour, almond meal, baking soda, baking powder, spices, and salt to another bowl. Whisk together until just incorporated, making sure no flour patches remain.
Slowly add your dry ingredients to your wet ingredients and mix together. Batter will be very thick—do not add any additional liquids.
Pour batter evenly into your prepared pan. Using a rubber spatula, spread into an even layer. Sprinkle streusel over pumpkin batter.
Bake the coffee cake for 30 to 40 minutes. Test for doneness by sticking a toothpick into the center of the cake. The toothpick will come out clean when the cake is done.
Allow to cool in the pan placed on a cooling rack for about 30 minutes. Pinch parchment paper and lift the cake out of the pan and transfer it directly to a cooling rack to cool completely, about 1to 3 hours. Slice and enjoy!
Cool completely, then store in an airtight container for up to 1 week at room temperature. If stored in the refrigerator, the topping will harden. Allow to soften at room temperature before enjoying, if desired. This coffee cake also freezes well. Slice first, then store in an airtight container in the freezer for up to 1 month. Allow to thaw at room temperature before enjoying.