I recommend making the jam the night before you want to make the macarons so that it has time to fully cool.
Add all of the blueberry jam ingredients to a large saucepan over medium-low heat. Allow the jam to simmer for 10-15 minutes, stirring frequently until thickened. Transfer the jam to a glass jar and allow to cool to room temperature.
Macarons
Sift together almond flour and powdered sugar into a large bowl and set aside.
In a stand mixer fitted with the whisk attachment, whisk the egg whites on medium speed until they become foamy, then add in the cream of tartar.
Turn the mixer up to medium-high and gradually add in the granulated sugar (make sure to add the sugar in very slowly so that the meringue doesn’t deflate).
Once all the sugar is added, turn the mixer to the highest setting and whisk until soft peaks form.
If desired, add in 1-2 drops of gel food coloring. Then continue to beat on high until stiff peaks form.
Using a spatula, gently fold in half of the dry ingredient mixture until fully combined. Then gently fold in the remaining dry ingredients. Continue to gently fold the batter until it reaches a “flowing lava” consistency. You should be able to make a “figure 8” with the batter before it breaks. As soon as you reach this consistency, stop mixing! If you over-mix the batter, the macarons won’t rise properly in the oven.
Line a large baking sheet with a silicone mat or parchment paper.
Scoop batter into a large piping bag fitted with a round tip. Hold the piping bag perpendicular to the silicone mat and pipe 1 inch rounds.
Pick up the baking sheet and drop it onto the counter 3-4 times to help bring any air bubbles to the surface of the macarons. Then use a toothpick to pop any visible air bubbles to make a smooth surface.
Let the macarons sit at room temperature for 30-40 minutes or until a skin forms on the surface. You should be able to lightly touch the surface without any batter being disturbed.
While the macarons are resting, preheat the oven to 300°F.
Bake one sheet at a time on the center rack of the oven for 13-15 minutes or when they do not move on their “feet” when touched.
Let the macarons cool completely on the baking sheet before removing them. They should easily peel off when fully cooled.
Match up your shells before filling, and then turn one side over.
Mascarpone Frosting
In a large bowl using a handheld or stand mixer, cream the mascarpone cheese, powdered sugar, and lemon juice until smooth. Then beat on high speed for 2-3 minutes until the frosting is super light and fluffy.
Once the macarons are cooled, pipe the mascarpone around the edge of the macaron shell. Then add a small scoop of blueberry jam in the center (about ½ teaspoon). Then top with the other shell.
Put the assembled blueberry mascarpone macarons in an airtight container, then place them in the fridge to mature for 12-24 hours. Store in the fridge for up to 5 days. However, for the best taste/texture, let them come up to room temperature for about an hour before eating! [Directly out of the fridge they can sometimes be super hard but at room temperature they're perfectly soft and tender!]
Notes
Tip: Clean mixing bowl and whisk attachment of your stand mixer with white vinegar to make sure there are no trace fats on either (if you skip this step, your meringue may not form).