Since the crème pâtissière will need time to set, make that first. In a medium bowl, whisk together sugar, egg yolks, and vanilla paste for 3–4 minutes until mixture is pale yellow and reaches the ribbon stage. Add flour and whisk until smooth.
In a medium saucepan, bring milk to a boil over medium heat, roughly 3 minutes.
Slowly pour a small amount of hot milk into egg yolk mixture while whisking constantly. (This gently raises the temperature of the eggs so that they don’t end up scrambled.) After egg yolk mixture has been tempered, pour in remaining milk while continuously whisking.
Return mixture to saucepan and bring to a boil over medium heat (3–5 minutes), whisking constantly to keep mixture from scorching and lumps from forming. The pastry cream is ready when large bubbles slowly come to the surface and pop through the thick, pudding-esque mixture.
Place a metal sieve over a heat-safe bowl, then strain crème pâtissière. Immediately press cling film directly onto surface. Let cool to room temperature, then transfer to fridge to chill for 2 hours or overnight.
When you’re ready to make the cookies, line baking sheets with parchment paper.
In a medium bowl, whisk together flour, Nutraberry Raspberry Seed Fiber, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with paddle attachment, beat butter and sugars on medium-high until light and fluffy.
Add eggs one at a time, mixing until thoroughly combined. Mix in vanilla.
Add 1/3 of the flour mixture to wet mixture, mixing until it’s almost all incorporated. Scrape down bowl. Repeat two more times with rest of flour mixture.
Fold in sprinkles, if you like. (We used ¼ cup (42g) of Sweetapolita’s Bees & Blooms Sprinkle Medley.)
Using a medium cookie scoop, form dough into balls that are about 2 tablespoons each. Place on prepared baking sheets, leaving 2" between each dough ball. Top with sprinkles, if desired. (We used flower quins from Beautiful Briny Sea.) Set baking sheets in refrigerator for 15 minutes.
Heat oven to 375°F. Bake cookies for 9–11 minutes or until cookies have set.
Cool on baking sheet for 10 minutes; transfer cookies to a wire rack to cool completely.
Once cookies are totally cool, it’s time to make sammiches! When you take the crème pâtissière out of the fridge, it will be very thick. Whisk for 2–3 minutes to loosen it up. Pipe crème pâtissière onto half of the cookies, leaving about ¼" between cream and edges.
Top with nekkid cookies, then gently press cookies together, spreading pastry cream almost to the edges.
Store in fridge until ready to enjoy. Eat within 3 days… as if they’ll last that long!