Preheat oven to 400ºF.
In a small bowl, mix the ground flaxseed with 6 Tbsp. water and let sit until thickened, about 5-7 minutes.
Sift together the flour, baking powder, and salt. Add the sugar and vanilla bean, then mix well.
Cut in the cold butter using a pastry cutter (or you can use your hands like I do!), and work the mixture until crumbly. It's ok if there are a few slightly larger, unincorporated chunks of butter.
Add the liquid ingredients (flax mixture and milk), and fold together gently until a dough comes together. It will be sticky! Chill the mixture for 10 minutes.
Dump half the dough onto a lightly-floured surface and pat into a circle. Spread the coffee streusel over the top of the dough in an even layer. Top with the remaining dough and pinch the edges to keep the streusel inside the dough. Using a serrated knife, cut across the dough into 8 equal triangles.
Transfer the scones onto a parchment-lined baking sheet, and chill an additional 10 minutes.
Right before you put the scones in the oven, brush with a small amount of milk and sprinkle with granulated sugar. Bake for 25-30 minutes until golden brown.