In a medium sized bowl, stir together all of the dry ingredients for crumb. Stir in the melted butter. Set the mixture aside while you make the cake batter.
In another medium sized bowl, stir together the flour, baking soda, baking powder, salt, and pumpkin spice.
In a large bowl, beat the sugar and butter together until smooth and fully combined.
To the sugar and butter mixture, beat in one egg until well combined. Then, add the second egg and beat it into the mixture until it is well combined also.
For these next steps - be careful not to go crazy with the mixing. Combine the ingredients JUST until they are mixed in. First, mix the pumpkin and the greek yogurt into the sugar/butter/eggs. Then, add in the flour and mix until it is just barely combined - lumps are totally fine and encouraged even.
Spray a 9x13 glass pan with cooking spray. Scrape the cake batter into the pan. Top with the crumb mixture, breaking up any clumps larger than a grape ( but leaving the clumps smaller than grapes to fall where they may ). Don't press the crumb into the cake batter.
Bake at 350 F for 50 minutes.