Preheat oven to 160°C. Grease and line a 20 x 20cm tin.
To make the brownie layer, place butter into the Thermomix bowl. Melt 2 min/70°C/speed 1
Add sugar, cocoa, salt, vanilla, orange zest and orange oil. Mix 20 sec/speed 4. Add eggs one at a time while mixing 20 sec/speed 4.
Add flour and combine 12 sec/Interval/dough setting. Add marmalade and stir through with a spatula. Scrape batter into prepared tin.
Bake for 30 minutes. Cool in tin.
To make the Buttercream, place all ingredients into the Thermomix bowl. Combine 20 sec/speed 5. Scrape and repeat until light and fluffy.
Spread evenly over cold brownie. Garnish with chocolate curls/shards and a glacé orange slices.