Preheat oven to 350°F. Line a 9" springform pan with parchment. (I find tracing a circle around the bottom of the pan and cutting to be the best method.)
In a large bowl add the butter, brown sugar, and white sugar. Use a hand mixer to beat until the butter and sugars are fully combined. (Since this recipe uses cold or only slightly softened butter, this may take longer than normal.)
Add the milk, ground flax, vanilla extract, and almond extract. Beat again until combined.
Add the flour, baking powder, and salt. Mix again until the flour is totally incorporated into the wet ingredients. Do not over mix.
Add the chocolate chips and sprinkles, if using. Mix again, for 10-20 seconds, just to combine.
Transfer the dough to the prepared pan. Press evenly into a flat layer.
Bake for 22 minutes for a soft cake or 25 minutes for a more crunchy cake. Let the cake cool completely. (I recommend chilling the cake before frosting for the best flavor.)
Make the frosting (see my vegan buttercream frosting post for full notes). In a medium bowl add the softened butter and beat until smooth. Add the vanilla and powdered sugar and mix on low speed until the frosting comes together. If you want to add coloring, remove ⅓ cup of the frosting and place in a separate bowl. Add the beetroot powder (or other coloring) and mix until incorporated.
To frost like I did, add both colors to separate sides of a piping bag and pipe two rows onto the circumference of the cake. Alternatively, you can cover the entire cake in frosting and use the colored portion to pipe a single row around the edge.