Combine all purpose flour, buckwheat flour, dark brown sugar and salt. Whisk until fully combined and no clumps remain.Add chopped walnuts and butter and mix with your fingers or a fork until crumbles form and no dry flour remains.Place the streusel in the fridge while you make the buckle.Note: Cold streusel maintains its shape better in the oven!
Preheat your oven to 350 degrees. Rub a 6 inch pan with a few drops of olive oil and line parchment on the bottom of the pan and a thin strip of parchment to create a hang. Set aside.Combine cake flour, baking soda, baking powder salt and whisk until incorporated. Set aside.In a clean bowl, beat the softened butter along with dark brown sugar and white sugar until light and fluffy. About two minutes.Add in the egg and vanilla extract and beat until combined.Beat in the skyr until combined.Add the flour mixture to the wet mixture and gently fold in using a spatula. Fold until the mixture is smooth.Fold in the frozen blueberries until just combined.Note: It’s okay if your batter turns a little blue.Pour the buckle batter into your lined cake pan. Use an offset spatula to even out the top.Pull the streusel out of the fridge and sprinkle it over the buckle batter evenly. Gently press the streusel crumbs into the buckle.Bake the buckle for about 40 - 45 minutes or just until an inserted toothpick comes out clear. Let the buckle cool for for a few minutes.
Whisk vanilla extract, powdered sugar and cold heavy cream until thick and fluffy.Fold in the blueberry skyr into the whipped cream until just combined.
Microwave the frozen blueberries along with the sugar until the blueberries are warm and jammy.
Once cooled, pull the buckle out by the parchment hang and serve warm.Serve your buckle nice warm along with the blueberry skyr cream, blueberry compote, fresh blueberries and some powdered sugar! Slice and enjoy!