Preheat oven to 350°F. Line an 8x11" baking sheet with parchment paper.
Beat butter and sugar in a large mixing bowl.
Add ground flax, milk, and almond extract. Mix until combined.
Add flour, baking powder, and salt (if using). Mix to form a dough. You can bake straight away or chill the dough if you're not ready to bake yet.
Spread dough into your lined baking sheet. Make sure the dough is evenly spread out. Use a spatula or the back of a large measuring cup to flatten out the surface of the dough. Scatter sprinkles on top (if using) and gently press them into the dough.
Bake for 18 minutes, until the edges just start to brown. The center will still feel soft but it will firm up as it cools. Let cool for 10 minutes, then remove from the pan and cut into squares. Transfer cookies to a wire rack to finish cooling.
Notes
Butter: If you don't have vegan butter/margarine, you can substitute with: 3 tbsp coconut oil, 2 tbsp vegetable oil, 1 tbsp soy milk.To make cut-out cookies: After mixing the dough, roll the dough out on parchment paper to about 1/4-inch thickness. Cover with plastic wrap and chill in the fridge for 1-2 hours. When ready to bake, cut out dough with cookie cutters and place on a parchment or silicone lined baking sheet, leaving an inch of space between each cookie. Bake 8-12 minutes, depending on the size of your cookies. Remove when they are lightly browned on the edges. Cool on the baking sheet for 5 minutes, then transfer to a wire rack to finish cooling.