Separate the two eggs, placing the yolks in a small bowl. Save the egg whites for another recipe. Line two baking sheets with parchment paper.

In a medium mixing bowl, whisk together the flour, cocoa powder, salt, and baking powder until well combined. Set aside.

In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together for 1–2 minutes until light and fluffy. Add the egg yolks and vanilla extract, mixing until well combined. Scrape down the sides and bottom of the bowl with a spatula, then give everything one final mix.

Add half of the dry ingredients to the butter mixture and gently fold together using a spatula. Add the remaining dry ingredients and continue folding until the chocolate cookie dough comes together. Avoid overmixing.

Use a small cookie scoop (about 1 tablespoon) to portion the dough. Roll each portion into a ball with your hands. Place the balls on the prepared baking sheets, leaving about 2 inches between each cookie. Using the back of a 1/2-teaspoon measuring spoon or your thumb, gently press an indentation into the centre of each cookie. Smooth any cracks around the edges with your fingers.

Place the baking sheets in the refrigerator for 30 minutes to chill the dough. While the cookies are chilling, preheat your oven to 350°F.

Bake the chilled cookies for 9–11 minutes. Avoid overbaking to keep the cookies soft and chewy. The edges should look set while the centres are still a little soft. If the thumbprint centres puff up during baking, gently press them down again while the cookies are still warm. Cool on the baking sheets for a few minutes before transferring to a wire rack to cool completely.

Place the semi-sweet chocolate chips and heavy whipping cream in a microwave-safe bowl. Microwave for 30 seconds, then stir well. Continue heating in 15-second intervals, stirring between each, until the chocolate is completely melted and the ganache is smooth and creamy.

Let the ganache cool for about 10–15 minutes, stirring occasionally, until it thickens enough to spoon into the thumbprint centres without overflowing.

Using a small spoon, fill the thumbprint centre of each cooled cookie with chocolate ganache. Add red, green, and white sprinkles for Christmas (or choose colours to match your occasion). Add the sprinkles while the ganache is still soft so they stick to the chocolate. Allow the ganache to set before serving.