To make your own brioche, look up at Brioche Recipe. Trim and slice into 3''/7.6cm diameter and 1''/2.5cm thick wheels. Save scraps for bread pudding.
Combine powdered sugar, almond meal and cornstarch together and set aside. Meanwhile in a stand mixer or by hand, cream the softened butter. Add dry ingredients, then the egg and rum. Use at room temperature.
Bring to a boil, and add flavoring. Use syrup at 160ºF/70ºC.
Soak brioche in syrup and flip a few times during soaking. Transfer onto a silicone mat, or parchment paper. Top with the almond cream within 1/4''/5mm off the edges. Dip in slivered almonds and dust with powdered sugar.
Bake bostock at 400ºF/200ºC for 15 to 20 min. Best served at room temperature. Bon appétit!