My macarons look great but then when I bite into them they are hallow-ish and crunchy/flaky. What am I doing wrong?

I did mature them in the fridge, and this is the recipe I used:
– 136 grams egg whites
– 136 grams granulated sugar
– 163 grams almond flour
– 163 grams powdered sugar
Process:
– Wipe down your mixing bowl, whisk, silicone mat, spatula, and piping tip with white vinegar. This removes any oils or residue that can ruin your merengue.
– Separate your eggs and let egg whites sit at room temperature for 15 minutes.
– Weigh the granulated sugar. Add 5 grams egg white powder (optional) and combine.
– Sift the almond flour and powdered sugar and combine with a whisk.
– Whisk egg whites on low speed until frothy bubbles form.
– Add in half of the sugar and egg white powder mixture and mix on low speed for a few minutes
– Add in half of the sugar and egg white powder mixture and mix on low speed for a few minutes.
– Add in the rest of the sugar and turn the mixer up to speed 4 for 10 minutes.
– Increase the speed to 6 for 3 minutes.
– Remove the whisk and swirl it around the bowl to check for stiff peaks.
– Add gel color and mix to combine (do not over mix).
– Pour in half of the almond flour and powdered sugar mixture.
– Gently combine until the dry ingredients are incorporated into the merengue.
– Add the rest of the dry ingredients and continue to incorporate.
– Use your spatula to press the batter against the side of the bowl.
– Continue this process until the batter flows off the spatula like a ribbon.
– Pour the batter into a piping bag fitted with a #12 piping tip.
– Pipe the batter onto a baking tray lined with a silicone mat or parchment paper. If your tray has sides, flip it upside down to allow for better airflow.
– Tap the bottom of the tray to pop air bubbles.
– Use a toothpick to pop any bubbles on the surface.
– Leave the tray to rest at room temperature for 15 minutes.
– Bake at 300F for 18-20 mins
– Cool completely before removing them from the mat.