Make the Pretzel Granita (Start 4+ Hours Ahead):
Combine water, torn pretzels, brown sugar, and salt in a medium saucepan. Bring to a simmer over medium heat.
Cook, stirring occasionally, until pretzels have completely broken down and liquid is deeply flavored, about 20 minutes.
Strain through a fine-mesh sieve, pressing on solids to extract all liquid. Discard pretzel solids.
Stir malt syrup and vanilla into the strained liquid. Taste and adjust salt—it should be noticeably salty-sweet, as freezing dulls flavors.
Pour into a shallow 9x13-inch baking dish. Freeze until edges begin to set, about 1 hour.
Using a fork, scrape the frozen edges toward the center. Return to freezer.
Repeat scraping every 30-45 minutes until entire mixture is fluffy, icy crystals—about 3-4 hours total. Cover and keep frozen until serving.

Cook the Barley:
Rinse pearl barley under cold water. Combine with 1½ cups water in a small saucepan. Bring to a boil, reduce heat, cover, and simmer until tender but still slightly chewy, 25-30 minutes.
Drain any excess liquid. Spread barley on a plate to cool completely.

Make the Pretzel Crumble:
Preheat oven to 325°F (165°C). Line a small baking sheet with parchment.
Toss crushed pretzels with melted butter, brown sugar, and flaky salt.
Spread on prepared sheet. Bake 10-12 minutes, stirring halfway, until golden and fragrant. Cool completely—it will crisp as it cools. Set aside.

Make the Barley Pretzel Cake:
Increase oven to 350°F (175°C). Grease a 9-inch square baking pan and line with parchment.
Pulse cooled barley in a food processor until coarsely chopped—you want texture, not paste. Transfer to a bowl.
Add crushed pretzels, flour, baking powder, and salt to the barley. Whisk to combine.
In a stand mixer with paddle attachment, cream butter and brown sugar until light and fluffy, 3-4 minutes.
Add eggs one at a time, beating well after each addition.
Mix in milk, malt syrup, and vanilla until combined.
Add barley-pretzel flour mixture and mix on low just until combined—do not overmix.
Spread batter in prepared pan, smoothing the top.
Bake 25-30 minutes until golden, springy to touch, and a toothpick inserted in center comes out clean.
Cool in pan 10 minutes, then turn out onto a wire rack. Cool completely before cutting.

Make the Honey Mustard Sabayon (Immediately Before Serving):
Combine egg yolks, honey, both mustards, wine, salt, and cayenne in a medium heatproof bowl.
Set bowl over a pan of simmering water (bowl should not touch water). Whisk constantly and vigorously.
Continue whisking until mixture triples in volume, becomes pale and thick, and holds a ribbon trail when whisk is lifted, 6-8 minutes. The sabayon should reach 160°F (71°C).
Remove from heat immediately. If using butter for extra richness, whisk in cold butter one piece at a time until incorporated.
Sabayon must be served immediately or kept warm over barely simmering water—it will deflate and separate if it sits. Whisk occasionally to maintain texture.

To Plate:
Cut cake into elegant portions—rectangles, squares, or use a ring mold for circles.
Place cake slightly off-center on each plate.
Scrape pretzel granita with a fork to fluff, then mound a generous portion beside or atop the cake.
Spoon warm honey mustard sabayon over and around the cake—it should pool slightly.
Scatter pretzel crumble for textural contrast.
Finish with flaky sea salt, a drizzle of honey, fresh thyme if using, and edible flowers.
Serve immediately while sabayon is warm and granita is icy.