Preheat the oven to 350°F. Butter a 9x5 inch loaf pan and set aside.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl or the bowl of a stand mixer, beat the butter on high speed until creamy (about 1 minute). Add the sugar and beat on high for another minute until well combined.
Turn the mixer on low speed and slowly add the eggs, one at a time, followed by the vanilla. Then beat on medium-high speed until the ingredients are well combined.
Turn the mixer back to low speed and slowly add the flour mixture. Beat the batter just until all of the flour is incorporated, being careful not to over mix. Stir in crushed Unna Bakery Oat Cookies.
Pour the batter into the loaf pan and bake for about one hour, until top of cake is golden brown and a stick inserted into the center of the loaf comes out clean.
Set the pan on a wire rack and allow the cake to cool completely in the pan. Once cool, run a butter knife around the edge of the pan to loosen the cake, then flip the pan over onto the wire rack to release the cake.
Cut the cake into ½ inch wide slices. You should have 18 slices. Using a 1 ½ inch circular cookie cutter, cut 2 small circles from each slice of cake (except for the two ends). You should now have 32 small circles.
Spread 1 tsp. of the orange marmalade atop half of the little cakes. Top each with the remaining cakes.
In a small bowl, mix the powdered sugar and milk to create a glaze. The glaze should be pourable but thick. If your glaze is too runny, add a little more powdered sugar. If it's too stiff, add a tiny bit more milk.
Pour the glaze over each petit four and quickly spread the glaze over the top with the back of a spoon. Some glaze may drip down the sides, but the sides should not be completely coated with glaze. Garnish with edible flowers.