Begin by making the chocolate ganache frosting. Combine the heavy cream and bittersweet chocolate in a medium saucepan or a microwaveable bowl. Heat on medium low until chips are melted and can be stirred smooth. If using the microwave, heat at 30 second intervals until completely melted and mixture can be stirred smooth. Add butter in and stir until melted. Add the vanilla. Place in the fridge for 30 minutes while you make the cookies.
Preheat the oven to 375 degrees.
Cream the butter and sugar in the bowl of a stand mixer (or hand mixer) for about 2 minutes, until fluffy. Add the vanilla, almond extract, and egg. Beat until well combined.
In a separate bowl, sift in the flour, cornstarch, baking soda, and salt. Slowly add the flour mixture to the butter mixture, and continue on low speed until it’s all added and combined. Mix in the sprinkles (don’t overmix or the sprinkles will start to bleed into the dough).
Form cookie dough into small balls, about a teaspoon in size. Place cookie dough balls on the prepared baking sheet. Bake for 8-10 minutes or until the cookies are light golden brown around the edges. The cookies will harden as they cool, so don't over bake. Cool cookies on a wire rack.
While the cookies are cooking, whip the ganache frosting. Place the ganache in the bowl of a stand mixer (or use a hand mixer) and whip on high until the color lightens and the ganache has the texture of frosting.
To make the sandwich cookies, pipe (using a pastry bag or plastic bag) small dollops of frosting on a cookie and sandwich another cookie on top. Store cookies in an air-tight container for up to 3 days.