Preheat the oven to 300°F and line 2 baking sheets with parchment paper, Teflon sheets, or silicone baking mats.
Sift the almond flour and 65 g powdered sugar together twice into a medium bowl, discarding any coarse bits left in the sieve.
In a clean, dry mixing bowl, combine the egg whites, granulated sugar, cream of tartar, and salt.
Using a stand mixer fitted with the whisk attachment or a hand mixer, beat on medium speed until soft peaks form, then continue to beat until stiff, glossy peaks hold their shape.
Sift the almond flour mixture over the meringue.
Using a silicone spatula, gently fold the dry ingredients into the meringue, pressing some of the air out against the side of the bowl, until the batter flows in a thick ribbon and can form a figure eight that disappears back into the batter in about 10 seconds.
Gently fold in 3 tablespoons of nonpareil sprinkles with a few light turns, stopping as soon as they are evenly distributed.
Transfer the batter to a pastry bag fitted with a round piping tip.
Pipe 1.5-inch rounds onto the prepared baking sheets, spacing them about 1 inch apart.
Firmly tap the baking sheets on the counter several times to release air bubbles, then use a toothpick to pop any visible large bubbles on the surface.
Lightly sprinkle a few additional sprinkles on top of each piped shell.
Let the shells rest at room temperature for about 15 minutes, or until the tops look slightly dull and feel dry to the touch.
Bake one sheet at a time on the middle rack for 15 to 18 minutes, or until the shells are set and the feet are firm and do not shift when gently nudged.
Remove from the oven and let the macaron shells cool completely on the baking sheets before lifting them off.
While the shells cool, beat the softened butter and 50 g powdered sugar together in a mixing bowl for about 2 minutes, until light and fluffy.
Melt the white chocolate chips in a microwave-safe bowl at 50% power in 20- to 30-second intervals, stirring between each, until smooth. Let cool slightly.
Add the melted white chocolate and vanilla bean paste to the butter mixture and beat until smooth and creamy.
Stir in 2 teaspoons of nonpareil sprinkles, then transfer the filling to a clean pastry bag.
Pair the cooled macaron shells by similar size and arrange them with the flat sides facing up.
Pipe a small mound of filling onto the center of half of the shells.
Gently place the remaining shells on top of the filling and press lightly until the filling spreads just to the edges.
Place the filled macarons in a single layer in an airtight container, separating layers with parchment if needed, and refrigerate for at least 24 hours to mature.
Bring the macarons to room temperature for about 30 minutes before serving.
Store leftover macarons in an airtight container in the refrigerator for up to 5 days or freeze for up to 5 months.