Preheat your oven to 350°F (175°C). Line your baking pan with parchment paper.
In a large mixing bowl, whisk together the whole wheat flour, oats, cinnamon, and kosher salt.
Grate the cold unsalted butter into the dry mixture and work it into the dry ingredients until it resembles coarse crumbs.
In a separate bowl, combine the egg yolks, plain Greek yogurt, honey, and vanilla extract. Whisk until smooth.
Pour the wet mixture into the dry ingredients and stir until just combined.
Spread half of the mixture into the prepared baking pan, press down firmly. Spread the blueberry jam over the crust, then sprinkle the remaining mixture on top.
Brush the top with buttermilk and bake for 25-30 minutes until golden brown.
Allow to cool for 10 minutes before lifting out and letting cool completely on a wire rack.